doc: add clarification on birthtime in fs stat

Clarifies the possibility of birthtime in the fs stat
object being greater than atime or mtime when not available
in the filesystem (see issue for further info).

Fixes: nodejs/node#2222
PR-URL: nodejs/node#5479
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/fs.markdown

@@ -190,10 +190,11 @@ The times in the stat object have the following semantics:
 * `birthtime` "Birth Time" -  Time of file creation. Set once when the
   file is created.  On filesystems where birthtime is not available,
   this field may instead hold either the `ctime` or
-  `1970-01-01T00:00Z` (ie, unix epoch timestamp `0`).  On Darwin and
-  other FreeBSD variants, also set if the `atime` is explicitly set to
-  an earlier value than the current `birthtime` using the `utimes(2)`
-  system call.
+  `1970-01-01T00:00Z` (ie, unix epoch timestamp `0`). Note that this
+  value may be greater than `atime` or `mtime` in this case. On Darwin
+  and other FreeBSD variants, also set if the `atime` is explicitly
+  set to an earlier value than the current `birthtime` using the
+  `utimes(2)` system call.
 
 Prior to Node v0.12, the `ctime` held the `birthtime` on Windows
 systems.  Note that as of v0.12, `ctime` is not "creation time", and