Обсуждение: pgsql: Update mmgr's README to mention BumpContext

Update mmgr's README to mention BumpContext

Oversight in 29f6a959c.

In passing, since we now have 4 memory context types to choose from,
provide a brief overview of the specialities of each memory context
type.

Reported-by: Amul Sul
Author: Amul Sul, David Rowley
Discussion: https://postgr.es/m/CAAJ_b94U2s9nHh--DEK=sPEZUQ+x7vQJ7529fF8UAH97QJ9NXg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/58cf2e120e8aee2daed5a9b9ee4471574f712b3b

Modified Files
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src/backend/utils/mmgr/README | 45 +++++++++++++++++++++++++------------------
1 file changed, 26 insertions(+), 19 deletions(-)