doc: update fs.realpath documentation

PR-URL: nodejs#48170
Fixes: nodejs#45067
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: sinkhaha

doc/api/fs.md

@@ -4115,6 +4115,9 @@ the path returned will be passed as a {Buffer} object.
 If `path` resolves to a socket or a pipe, the function will return a system
 dependent name for that object.
 
+A path that does not exist results in an ENOENT error.
+`error.path` is the absolute file path.
+
 ### `fs.realpath.native(path[, options], callback)`
 
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