doc: document sticky-EOF issue

doc: document sticky-EOF issue

* doc/posix-functions/fgetc.texi (fgetc):
* doc/posix-functions/fgets.texi (fgets):
* doc/posix-functions/fread.texi (fread):
* doc/posix-functions/fscanf.texi (fscanf):
* doc/posix-functions/getc.texi (getc):
* doc/posix-functions/getchar.texi (getchar):
* doc/posix-functions/scanf.texi (scanf):
Mention that glibc and default Solaris do not conform to
C99 and POSIX-2001 or later, with respect to how getchar
etc. behave when feof reports nonzero.