Update the documentation for FielSystemEntity.resolveSymbolicLinks

Update the documentation for FielSystemEntity.resolveSymbolicLinks

The platform difference was the opposite of what was documented.

With the following directory structure

 /tmp/xxx
        a/b
        x -> a/b

resolving x/.. on Windows results in /tmp/xxx and on Linux results in
/tmp/xxx/a. Resolving just x results in /tmp/xxx/a/b on both platforms.

R=whesse@google.com, lrn@google.com
BUG=http://dartbug.com/21967

Committed: https://code.google.com/p/dart/source/detail?r=42725

[Bill Hesse, 2015-01-05 15:49:29]

LGTM.  This behavior is tested in
standalone/io/resolve_symbolic_links_test.dart, and the test, and comments, are
correct, and agree with the new documentation.

[Søren Gjesse, 2015-01-09 08:20:16]

Committed patchset #1 (id:1) manually as 42725 (presubmit successful).