g_str_to_ascii(): a couple of minor tweaks

Add a precondition guard and use g_string_sized_new() to avoid some
reallocations.
gsource: document priority of child sources

Add a note to the documentation that child sources cannot have their priority
changed independently from their parent.  Add a g_return_if_fail() to the
public API in order to enforce this.

This was already a reality due to the check in
g_source_set_priority_unlocked(), but it was never explicitly documented.

https://bugzilla.gnome.org/show_bug.cgi?id=724706