doc: clarify symlink resolution for __filename

Make it more explicit that symlinks are resolved in `__filename`.

Refs: nodejs#22602 (comment)
PR-URL: nodejs#24587
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/api/modules.md

@@ -491,8 +491,8 @@ added: v0.0.1
 
 * {string}
 
-The file name of the current module. This is the resolved absolute path of the
-current module file.
+The file name of the current module. This is the current module file's absolute
+path with symlinks resolved.
 
 For a main program this is not necessarily the same as the file name used in the
 command line.