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 
Reviewed-by: Matheus Alcantara 
Reviewed-by: Christoph Berg 
Discussion: https://www.postgresql.org/message-id/6B5BF07B-8A21-48E3-858C-1DC22F3A28B4@justatheory.com