pgsql: doc: Flesh out extension docs for the "prefix" make variable

doc: Flesh out extension docs for the "prefix" make variable

The variable is a bit magical in how it requires "postgresql" or
"pgsql" to be part of the path, and files end up in its "share" and
"lib" subdirectories.  So mention all that and show an example of
setting "extension_control_path" and "dynamic_library_path" to use
those locations.

Author: David E. Wheeler <david@justatheory.com>
Reviewed-by: Matheus Alcantara <matheusssilv97@gmail.com>
Reviewed-by: Christoph Berg <myon@debian.org>
Discussion: https://www.postgresql.org/message-id/6B5BF07B-8A21-48E3-858C-1DC22F3A28B4@justatheory.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/0064020680c1293ec6ce9fe2588e33d6aebdb015

Modified Files
--------------
doc/src/sgml/extend.sgml | 34 ++++++++++++++++++++++++++++++----
1 file changed, 30 insertions(+), 4 deletions(-)