Document '%F' format specifier

The '%F' format specifier was implemented in commit 6c46718f on
2000-08-23, but remains undocumented in the manual.
https://stackoverflow.com/questions/75157669/format-specifier-f-missing-from-glibcs-documentation



Fix that.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>