docs: Amend the docs for g_str_equal() to reflect current general usage

People do (and should) use g_str_equal() for string comparisons outside of hash tables, because it’s easier to read than strcmp (str1, str2) == 0. That should not be discouraged.

However, we should still be careful to point out that g_str_equal() is not NULL-safe, and g_strcmp0() is.

Signed-off-by: Philip Withnall withnall@endlessm.com