Previously only Gson.fromJson(..., Type) existed which is however not
type-safe since the generic type parameter T used for the return type is
not bound.
Since these methods are often used in the form
  gson.fromJson(..., new TypeToken<...>(){}.getType())
this commit now adds overloads which accept a TypeToken and are therefore
more type-safe.

Additional changes:
- Fixed some grammar mistakes
- Added javadoc @see tags
- Consistently write "JSON" in uppercase
- More precise placement of @SuppressWarnings("unchecked")

The newly added documentation deliberately does not state which exception
is thrown because Gson.assertFullConsumption could throw either a
JsonIOException or a JsonSyntaxException.

Since the actual implementation of Gson.fromJson is TypeToken based, the
TypeToken variant overloads are now the "main" implementation and the other
overloads delegate to them.
Previously the Type variant overloads were the "main" implementation which
caused `TypeToken.getType()` followed by `TypeToken.get(...)` when the
TypeToken variant overloads were used.

To be consistent with the other Gson.fromJson(..., Class) overloads the
method should cast the result.

As noticed during review adding this method is source incompatible.