Fixup existing docs and add missing docs.

Testing Done:
CI went green here:
  https://travis-ci.org/pantsbuild/pants/builds/77389176

Bugs closed: 2074

Reviewed at https://rbcommons.com/s/twitter/r/2708/

contrib/go/src/python/pants/contrib/go/targets/BUILD

@@ -12,15 +12,15 @@ python_library(
   name='go_binary',
   sources=['go_binary.py'],
   dependencies=[
-    'contrib/go/src/python/pants/contrib/go/targets:go_local_source',
+    ':go_local_source',
   ],
 )
 
 python_library(
   name='go_library',
   sources=['go_library.py'],
   dependencies=[
-    'contrib/go/src/python/pants/contrib/go/targets:go_local_source',
+    ':go_local_source',
   ],
 )
 

contrib/go/src/python/pants/contrib/go/targets/go_binary.py

@@ -9,4 +9,4 @@
 
 
 class GoBinary(GoLocalSource):
-  pass
+  """Represents a local Go main package."""

contrib/go/src/python/pants/contrib/go/targets/go_library.py

@@ -9,4 +9,4 @@
 
 
 class GoLibrary(GoLocalSource):
-  pass
+  """Represents a local Go package."""

contrib/go/src/python/pants/contrib/go/targets/go_local_source.py

@@ -17,25 +17,13 @@ class GoLocalSource(GoTarget):
 
   @classmethod
   def is_go_source(cls, path):
+    """Returns `True` if the file at the given `path` is a go source file."""
     return path.endswith('.go') and os.path.isfile(path)
 
   @classmethod
   def local_import_path(cls, source_root, address):
     """Returns the Go import path for the given address housed under the given source root.
 
-    A remote package path is the portion of the remote Go package's import path after the remote
-    root path.
-
-    For example, the remote import path 'https://github.com/bitly/go-simplejson' has
-    a remote root of 'https://github.com/bitly/go-simplejson' and there is only 1 package
-    in that remote root.  The package path in this case is '' or '.' and is normalized
-    to ''.
-
-    Some remote roots have no root package and others have both a root and sub-packages.  The
-    remote root of 'github.com/docker/docker' is an example of the former.  One of the packages
-    you might import from it is 'github.com/docker/docker/daemon/events' and that package has a
-    normalized remote package path of 'daemon/events'.
-
     :param string source_root: The path of the source root the address is found within.
     :param address: The target address of a GoLocalSource target.
     :type: :class:`pants.base.address.Address`
@@ -55,4 +43,5 @@ def __init__(self, address=None, payload=None, **kwargs):
 
   @property
   def import_path(self):
+    """The import path as used in import statements in `.go` source files."""
     return self.local_import_path(self.target_base, self.address)