doc: Document pg_encoding_to_char() and pg_char_to_encoding().

doc: Document pg_encoding_to_char() and pg_char_to_encoding().

Previously both functions were not described anywhere in the docs.
But since they have been around since 7.0 and mentioned in the description
for system catalog like pg_database, it's reasonable to add short
descriptions for them.

Author: Ian Lawrence Barwick
Reviewed-by: Laurenz Albe, Fujii Masao
Discussion: https://postgr.es/m/CAB8KJ=infievn4q1N4X7Vx8w4_RMPPG0pLvxhSDjy5WQOSHW9g@mail.gmail.com