gquark: Document that some functions can’t initialise C++ globals

Any function which requires g_quark_init() to have been called first cannot be called before the library constructors have finished running. In particular, this means that g_quark_from_static_string() or g_intern_static_string() can’t be used to initialize C++ globals.

Do this, rather than adding a conditional call to g_quark_init() to all these functions, because such a call was previously removed from the functions to improve performance (quarks are used a lot in the implementation of GObject for properties and signals). That’s the reason why g_quark_init() was originally moved out to a library constructor.

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

Fixes: #1177 (closed)