clean up docs for log_statement_sample_rate

Following the dicussion here,

https://www.postgresql.org/message-id/flat/CAD21AoB9%2By8N4%2BFan-ne-_7J5yTybPttxeVKfwUocKp4zT1vNQ%40mail.gmail.com#90a8316b1e643532e1cdb352c91c22a7

I'm proposing these changes to clean up docs for previous (more or less
unrelated) commit.

From 15d42c5a8f2f811a7add3e4179edcc1f7cd291f7 Mon Sep 17 00:00:00 2001
From: Justin Pryzby <pryzbyj@telsasoft.com>
Date: Thu, 28 Mar 2019 08:53:26 -0500
Subject: [PATCH v1] Clean up docs for log_statement_sample_rate..

..which was added at commit 88bdbd3f746049834ae3cc972e6e650586ec3c9d
---
 doc/src/sgml/config.sgml                      | 18 +++++++++---------
 src/backend/utils/misc/guc.c                  |  4 ++--
 src/backend/utils/misc/postgresql.conf.sample |  6 +++---
 3 files changed, 14 insertions(+), 14 deletions(-)

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index d383de2..4019a31 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -5786,9 +5786,9 @@ local0.*    /var/log/postgresql
          Causes the duration of each completed statement to be logged
          if the statement ran for at least the specified number of
          milliseconds, modulated by <varname>log_statement_sample_rate</varname>.
-         Setting this to zero prints all statement durations. Minus-one (the default)
-         disables logging statement durations. For example, if you set it to
-         <literal>250ms</literal> then all SQL statements that run 250ms or longer
+         Setting this to zero prints all statement durations. <literal>-1</literal> (the default)
+         disables logging statements due to exceeding duration threshold. For example, if you set it to
+         <literal>250ms</literal>, then all SQL statements that run 250ms or longer
          will be logged. Enabling this parameter can be helpful in tracking down
          unoptimized queries in your applications.
          Only superusers can change this setting.
@@ -5824,13 +5824,13 @@ local0.*    /var/log/postgresql
       </term>
        <listitem>
         <para>
-         Determines the fraction of the statements that exceed
-         <xref linkend="guc-log-min-duration-statement"/> which to log.
-         The default is <literal>1</literal>, meaning log to all such
+         Determines the fraction of statements that exceed
+         <xref linkend="guc-log-min-duration-statement"/> to be logged.
+         The default is <literal>1</literal>, meaning log all such
          statements.
-         Setting this to zero disables logging, same as setting
+         Setting this to zero disables logging by duration, same as setting
          <varname>log_min_duration_statement</varname>
-         to minus-one. <varname>log_statement_sample_rate</varname>
+         to <literal>-1</literal>. <varname>log_statement_sample_rate</varname>
          is helpful when the traffic is too high to log all queries.
         </para>
        </listitem>
@@ -6083,7 +6083,7 @@ local0.*    /var/log/postgresql
 
        <note>
         <para>
-         The difference between setting this option and setting
+         The difference between enabling <varname>log_duration</varname> and setting
          <xref linkend="guc-log-min-duration-statement"/> to zero is that
          exceeding <varname>log_min_duration_statement</varname> forces the text of
          the query to be logged, but this option doesn't.  Thus, if
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index aa564d1..415cd78 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -3357,8 +3357,8 @@ static struct config_real ConfigureNamesReal[] =
 
     {
         {"log_statement_sample_rate", PGC_SUSET, LOGGING_WHEN,
-            gettext_noop("Fraction of statements over log_min_duration_statement to log."),
-            gettext_noop("If you only want a sample, use a value between 0 (never "
+            gettext_noop("Fraction of statements exceeding log_min_duration_statement to be logged."),
+            gettext_noop("If you only want a sample, use a value between 0.0 (never "
                          "log) and 1.0 (always log).")
         },
         &log_statement_sample_rate,
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index cccb5f1..684f5e7 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -489,9 +489,9 @@
                     # 0 logs all statement, > 0 logs only statements running at
                     # least this number of milliseconds.
 
-#log_statement_sample_rate = 1    # Fraction of logged statements over
-                    # log_min_duration_statement. 1.0 logs all statements,
-                    # 0 never logs.
+#log_statement_sample_rate = 1.0    # Fraction of logged statements exceeding
+                    # log_min_duration_statement to be logged
+                    # 1.0 logs all statements, 0.0 never logs
 
 # - What to Log -