Minor tweaks to API documentation and code comments.

mojo/public/c/system/types.h

Index: mojo/public/c/system/types.h
diff --git a/mojo/public/c/system/types.h b/mojo/public/c/system/types.h
index a88b024fe7eed15cc06d14742bac4653c6738523..f228384ad24d3e4872b44b05ff36bb65a324b78b 100644
--- a/mojo/public/c/system/types.h
+++ b/mojo/public/c/system/types.h
@@ -17,7 +17,16 @@
 // TODO(vtl): Notes: Use of undefined flags will lead to undefined behavior
 // (typically they'll be ignored), not necessarily an error.
 
-// |MojoTimeTicks|: Used to specify time ticks. Value is in microseconds.
+// |MojoTimeTicks|: The amount of time, in units of microseconds, that has

[viettrungluu, 2015/01/30 22:34:17]

How about, simply: "A time delta, in microseconds, the meaning of which is
source-dependent."

[ggowan, 2015/01/30 23:17:12]

I think that somebody who is coming here to look at this definition would
probably also appreciate a reference to the MojoGetTimeTicksNow() function,
which, practically speaking, is the only place values of this type are likely to
come from.

However, I'll just put the text you've asked for since it's definitely an
improvement over what's there now. If you agree a reference to the function is
appropriate, I'll be happy to add it, perhaps in a follow-up CL.

+// passed since some undefined point in the past. The values are only meaningful
+// relative to other values obtained from the same device without an intervening
+// system restart. Such values are guaranteed to be weakly
+// monotonically-increasing with the passage of real time, meaning they will not
+// always advance, but they will never go backwards (unlike time values with a
+// well-defined epoch).  Although the units are microseconds, the values should
+// not be assumed to be at that level of precision. The actual precision varies
+// depending on hardware and platform, and is typically in the range of ~1-15
+// ms.
 
 typedef int64_t MojoTimeTicks;