manual/stdio: Clarify putc and putwc

The manual entry for `putc' described what "most systems" do instead of
describing the glibc implementation and its guarantees.  This commit
fixes that by warning that putc may be implemented as a macro that
double-evaluates `stream', and removing the performance claim.

Even though the current `putc' implementation does not double-evaluate
`stream', offering this obscure guarantee as an extension to what
POSIX allows does not seem very useful.

The entry for `putwc' is also edited to bring it in line with `putc'.
Reviewed-by: Florian Weimer <fweimer@redhat.com>

(cherry picked from commit 10de4a47)