Rename validate_relation_kind()

Rename validate_relation_kind()

There are three static definitions of validate_relation_kind() in the
codebase, one each in table.c, indexam.c and sequence.c, validating that
the given relation is a table, an index or a sequence respectively.
The compiler knows which definition to use where because they are static.
But this could be confusing to a reader. Rename these functions so that
their names reflect the kind of relation they are validating. While at
it, also update the comments in table.c to clarify the definition of
table-like relkinds so that we don't have to maintain the exclusion list
as the set of relkinds undergoes changes.

Author: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Reviewed-by: Junwang Zhao <zhjwpku@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/6d3fef19-a420-4e11-8235-8ea534bf2080%40eisentraut.org