ENH: Allow unnamed return values in Returns section of doc string

Developers usually only need the type of a return value
followed by a brief description. However, in some cases
providing a name for a return value can make the documentation
clearer. This enhancement changes the format of the Returns
section such that the type is required, and the name is
optional:

    Returns
    -------
    int
        Description of anonymous integer return value.
    x : str
        Description of string return value named `x`.

With this change, if a colon is not present, then the entire
line is interpreted as the return type. In all other cases,
the Returns section is interpreted according to the current
rules.

Consistent with the current format, if a colon is present, then
the text to the left of the colon is interpreted as the name;
and the text to the right of the colon is interpreted as the
type. This makes the proposed change backwards compatible with
existing documentation.
Merge pull request #3468 from RelentlessIdiot/numpydoc_returns_unnamed

ENH: Allow unnamed return values in Returns section of doc string