Fix GH-10964: Improve man page about the built-in server

Author: alexandre-daubois

sapi/cli/php.1.in

@@ -92,6 +92,15 @@ point to a local address and port PHP will listen to HTTP requests on that addre
 .B docroot
 passed by the \-t option.
 .LP
+If a PHP file is provided to the command line when the
+built-in web server is used, it will be used as the router script. This script
+will be started at each HTTP request. The script output is returned to the
+browser, unless the router script returns the
+.B false
+value. If so, the built-in server falls back to the default behaviour, returning
+the requested resource as-is by looking up the files relative to the document
+root specified by the \-t option, if provided.
+.LP
 If none of \-r \-f \-B \-R \-F \-E or \-S is present but a single parameter is given
 then this parameter is taken as the filename to parse and execute (same as
 with \-f). If no parameter is present then the standard input is read and