pgsql: Doc: flesh out fmgr/README's description of context-node usage.

Doc: flesh out fmgr/README's description of context-node usage.

I wrote this to provide a home for a planned discussion of error
return conventions for non-error-throwing functions.  But it seems
useful as documentation of existing code no matter what becomes of
that proposal, so commit separately.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/29452de7341b0effbc275d7e139ade83cebda69f

Modified Files
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src/backend/utils/fmgr/README | 42 +++++++++++++++++++++++++++++++++++++-----
1 file changed, 37 insertions(+), 5 deletions(-)