hrtime.h: add note explaining current use of uint64_t [ci skip]

[ruby-core:88678]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64554 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

hrtime.h

@@ -8,7 +8,8 @@
 
 /*
  * Hi-res monotonic clock.  It is currently nsec resolution, which has over
- * 500 years of range (unsigned).
+ * 500 years of range (with an unsigned 64-bit integer).  Developers
+ * targeting small systems may try 32-bit and low-resolution (milliseconds).
  *
  * TBD: Is nsec even necessary? usec resolution seems enough for userspace
  * and it'll be suitable for use with devices lasting over 500,000 years