fs(3lua): init man pages

Initialise man pages for the fs module.

man/fs/fs.isdirectory.3lua.scd

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+FS.ISDIRECTORY(3lua) "3lua" "Callisto manual pages"
+
+# NAME
+*fs.isdirectory* - check whether a path points to a directory
+
+# SYNOPSIS
+*fs.isdirectory*(_path_: *string*)
+
+# DESCRIPTION
+*fs.isdirectory* returns whether _path_ points to a directory.
+
+If an error occurs,
+*fs.isdirectory* returns nil, an error message and
+a platform-dependent error code.
+Usually this only happens when passing a nonexistent path,
+or a path to a file which you do not have permission to read.
+See lstat(2) for a complete list.
+
+# EXAMPLES
+Check whether /bin is a directory:
+
+	if not fs.isdirectory("/bin") then
+		print("something's wrong...")
+	end
+
+# SEE ALSO
+callisto(3lua), fs(3lua), fs.isfile(3lua)