docs: Add missing GTest functions

The GTest message API is missing from the reference.
gsettings: Fix annotations
strfuncs: Make g_str_tokenize_and_fold introspectable
test default GSimpleAction activation

Test the default handling of the "activate" signal on GSimpleAction.

https://bugzilla.gnome.org/show_bug.cgi?id=722503
gsubprocess: Add missing transfer annotations to get_*_pipe()
gio: Fix some header/source parameter-naming mismatches
Assamese translation updated
glib: annotate some memory functions with allow-none