Fix documentation for conversion to bignums

Fix documentation for conversion to bignums

* src/xselect.c (selection_data_to_lisp_data):
* src/w32fns.c (Fw32_read_registry):
* src/process.c (Fprocess_id):
* src/font.c (Ffont_variation_glyphs, Finternal_char_font):
* src/fns.c (Fsafe_length):
* src/editfns.c (Fuser_uid, Fuser_real_uid, Fgroup_gid)
(Fgroup_real_gid, Femacs_pid):
* src/dired.c (Ffile_attributes):
* src/charset.c (Fencode_char): Update commentary and doc
strings for recent changes that produce bignums where
previously cons cells of integers were produced.