pgsql: Typo and doc fixups for memory context reporting

Typo and doc fixups for memory context reporting

This fixes comment and docs typos as well as a small documentation
change to make it clearer.  Found via post-commit review.

Author: Rahila Syed <rahilasyed90@gmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/CAH2L28vt16C9xTuK+K7QZvtA3kCNWXOEiT=gEekUw3Xxp9LVQw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f8c115a6cb6531ca86e7b0febd02698135827337

Modified Files
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doc/src/sgml/func.sgml        |  5 ++---
src/backend/utils/mmgr/mcxt.c | 14 ++++++++------
2 files changed, 10 insertions(+), 9 deletions(-)