manual/stdio: Further clarify putc, putwc, getc, and getwc

This is a follow-up to 10de4a47 that
reworded the manual entries for putc and putwc and removed any
performance claims.

This commit further clarifies these entries and brings getc and getwc in
line with the descriptions of putc and putwc, removing any performance
claims from them as well.
Reviewed-by: Florian Weimer <fweimer@redhat.com>