docstring: in python_binary(platforms=...) param doc, add an example

add an illustrative example to docstring. What I learned from pex-tool/pex#16

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/41992722

Bugs closed: 591

Reviewed at https://rbcommons.com/s/twitter/r/1386/

src/python/pants/backend/python/targets/python_binary.py

@@ -55,7 +55,11 @@ def __init__(self,
     :param repositories: a list of repositories to query for dependencies.
     :param indices: a list of indices to use for packages.
     :param ignore_errors: should we ignore inability to resolve dependencies?
-    :param platforms: extra platforms to target when building this binary.
+    :param platforms: extra platforms to target when building this binary. If this is, e.g.,
+      ``['current', 'linux-x86_64', 'macosx-10.4-x86_64']``, then when building the pex, then
+      for any platform-dependent modules, Pants will include ``egg``\s for Linux (64-bit Intel),
+      Mac OS X (version 10.4 or newer), and the current platform (whatever is being used when
+      making the PEX).
     :param compatibility: either a string or list of strings that represents
       interpreter compatibility for this target, using the Requirement-style format,
       e.g. ``'CPython>=3', or just ['>=2.7','<3']`` for requirements agnostic to interpreter class.