doc: Fix some descriptions related to pg_ident_file_mappings

doc: Fix some descriptions related to pg_ident_file_mappings

pg_ident_file_mappings.line_number was described as a line number in
pg_ident.conf for a "rule" number, but this should refer to a "map".
The same inconsistent term was used in the main paragraph describing the
view.

Extracted from a patch by the same author.  Issue introduced by
a2c8499 where this view has been added.

Author: Julien Rouhaud
Discussion: https://postgr.es/m/20221026031948.cbrnzgy5e7glsq2d@jrouhaud
Backpatch-through: 15