Bug 1609662: part 6) Update documentation of `Selection::GetIndicesForInterval`. r=smaug

Differential Revision: https://phabricator.services.mozilla.com/D60130

--HG--
extra : moz-landing-system : lando

dom/base/Selection.h

@@ -753,6 +753,9 @@ class Selection final : public nsSupportsWeakReference,
    * Works on the same principle as GetRangesForIntervalArray, however
    * instead this returns the indices into mRanges between which the
    * overlapping ranges lie.
+   *
+   * @param aStartIndex will be less or equal than aEndIndex.
+   * @param aEndIndex can be in [-1, mRanges.Length()].
    */
   nsresult GetIndicesForInterval(const nsINode* aBeginNode,
                                  int32_t aBeginOffset, const nsINode* aEndNode,