Enrich JavaDoc for IC#deleteSurroundingText().

As a preparation work for Bug 6526420, where we are going to
introduce a new variant of IC#deleteSurroundingText(), this CL attempts
to clarify about the expected behavior of IC#deleteSurroundingText().

With this CL, the expected behavior when the number of existing
characters is smaller than the number of characters to be deleted is
now documented by simply describing the current behavior of
BaseInputConnection, that is, IC#deleteSurroundingText() will delete
characters until it deletes requested number of characters in code units
or reaches to an end.

This is purely documentation work.  No behavior change is intended.

Bug: 6526420
Change-Id: I7280dec07e1c043bd26b3b12fd866b9d8b6186cc