Clarify on return value of IC#setImeConsumesInput()

This CL aims to clarify on what the return value of

  InputConnection#setImeConsumesInput()

means for both the IME authors and editor authors [1][2].

In short, the semantics is exactly the same as

  InputConnection#performSpellCheck()

hence this CL reuses its @return section.

Other than clarifying an API doc, this CL changes nothing.

 [1]: Ic04cacfd73f1f4cb254bb16caf6b04c00c91a318
      30fe6aa9
 [2]: I8ac1ea4d53f747b0086ed415ff90793dfc6155bc
      66b21086

Bug: 175362887
Fix: 194099171
Test: presubmit
Change-Id: Ibdfe81a9dd1448856797aaed2f14de1314da1cac