pgsql: Add documentation about calling version-1 C functions from C.

Add documentation about calling version-1 C functions from C.

This topic wasn't really covered before, so fill in some details.

Author: Florents Tselai <florents.tselai@gmail.com>
Reviewed-by: Pavel Stehule <pavel.stehule@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/90853055-5BBD-493D-91E5-721677C7C59B@gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/86e4efc52b664b60f9494e062bbf0454cabcb61f

Modified Files
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doc/src/sgml/xfunc.sgml   | 73 +++++++++++++++++++++++++++++++++++++++++++++++
src/tutorial/funcs.c      | 20 +++++++++++++
src/tutorial/funcs.source | 24 ++++++++++++----
3 files changed, 112 insertions(+), 5 deletions(-)