(u)alarm.3: Improve readability of setitmer(2) restrictions

Add separators to make the maximum number of (micro)seconds
in (u)alarm.3 more readable.

Sponsored by:	Klara, Inc.
Obtained from:	https://github.com/apple-oss-distributions/Libc
Differential Revision:	https://reviews.freebsd.org/D49712
Reviewed by:	markj

lib/libc/gen/alarm.3

@@ -63,7 +63,7 @@ Due to
 .Xr setitimer 2
 restriction the maximum number of
 .Fa seconds
-allowed is 100000000.
+allowed is 100,000,000.
 .Sh RETURN VALUES
 The return value of
 .Fn alarm

lib/libc/gen/ualarm.3

@@ -70,7 +70,7 @@ restriction the maximum number of
 .Fa microseconds
 and
 .Fa interval
-is limited to 100000000000000
+is limited to 100,000,000,000,000
 (in case this value fits in the unsigned integer).
 .Sh RETURN VALUES
 When the signal has successfully been caught,