For every function, document exactly what type it returns

e.g https://mariadb.com/kb/en/library/connection_id/
used to return MYSQL_LONG_LONG in 10.2 in result set metadata, but
and changed to MYSQL_LONG UNSIGNED in 10.3

We need to track the changes between the versions, so there are not many potentially breaking changes in the future.

as noted in MDEV-16347, changing types can break client programs
In 10.3, according to Bar, MDEV-12619 caused all functions that used to return 64bit now can return 32bit, and those are over 50, and this breaking change went almost undocumented.