pgsql: doc: clarify that auth. names are lower case and case-sensitive

doc:  clarify that auth. names are lower case and case-sensitive

This is true even for acronyms that are usually upper case, like LDAP.

Reported-by: Alvaro Herrera

Discussion: https://postgr.es/m/202205141521.2nodjabmsour@alvherre.pgsql

Backpatch-through: 10

Branch
------
REL_10_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/7fa2bdeda62e310c87713e8e48d2caaf452b401e

Modified Files
--------------
doc/src/sgml/client-auth.sgml | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)