[PATCH] Document that directly callable functions may use fn_extra

Hi all

I was a puzzled by src/backend/utils/fmgr/README and fmgr.h's descriptions
of fcinfo->flinfo->fn_extra (FmgrInfo.fn_extra) as they seem to conflict
with actual usage.

The docs suggest that fl_extra is for the use of function call handlers,
but in practice it's also used heavily by function implementations as a
cache space.

For example, SQL functions use fn_extra in init_sql_fcache, plpgsql uses it
in plpgsql_compile, but also most of the array functions use it for a type
cache.

I'm inclined to change the docs to say that functions called directly by
the fmgr may also use fn_extra (per existing practice). Handlers that wrap
functions usually called directly by the fmgr must save and restore
fn_extra or leave it untouched.

Trivial patch to do the above attached.

--
Craig Ringer http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services