process(3lua): init man pages

Initialise man pages for the process module.

man/process/process.signum.3lua.scd

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+PROCESS.SIGNUM(3lua) "3lua" "Callisto manual pages"
+
+# NAME
+*process.signum* - translate a signal's name into its numeric constant
+
+# SYNOPSIS
+*process.signum*(_sig_: *string*)
+
+# DESCRIPTION
+*process.signum* returns the numeric form of the signal name _sig_.
+The returned integer will likely differ across platforms.
+
+On many platforms, *SIGKILL* has the numeric value *9*;
+therefore, on those platforms _process.signum("SIGKILL")_
+would return *9*.
+
+If _sig_ is not the name of a valid signal,
+*process.signum* throws a Lua error.
+
+# EXAMPLES
+Print the value of *SIGUSR1*:
+
+	print("SIGUSR1 is " .. process.signum("SIGUSR1"))
+
+# SEE ALSO
+callisto(3lua), process(3lua)