[skip ci] Add link to FPM docs in FPM man page (#14853)

This removes obsolete link to php-fpm.org and redundant manual link in
favor of a dedicated FPM chapters.

Can be tested with:

    man sapi/fpm/php-fpm.8.in

Author: petk

sapi/fpm/php-fpm.8.in

@@ -175,16 +175,10 @@ The PHP-FPM CGI daemon will work well with most popular webservers, including Ap
 .PD 1
 .P
 .SH SEE ALSO
-The PHP-FPM website:
+For a more or less complete description of PHP-FPM look here:
 .PD 0
 .P
-.B https://php-fpm.org
-.PD 1
-.P
-For a more or less complete description of PHP look here:
-.PD 0
-.P
-.B https://www.php.net/manual/
+.B https://www.php.net/fpm
 .PD 1
 .SH BUGS
 You can view the list of known bugs or report any new bug you