libc: Document some more *_l() functions.

fgetwc_l()
getwc_l()
getwchar_l()

fputwc_l()
putwc_l()
putwchar_l()
libc: Document some more *_l() functions.

strtod_l()
strtof_l()
strtoimax_l()
strtol_l()
strtold_l()
strtoll_l()
strtoq_l()
strtoul_l()
strtoull_l()
strtoumax_l()
strtouq_l()
libc: Document some more *_l() functions.

wcstod_l()
wcstof_l()
wcstoimax_l()
wcstol_l()
wcstold_l()
wcstoll_l()
wcstoul_l()
wcstoull_l()
wcstoumax_l()
libc: Add xlocale(3) references to some manual pages.
libc: Document some more *_l() functions.

fgetws_l()
fputws_l()
mblen_l()
mbrlen_l()