Release v1.6.0 (#324)

* Update version and changelog for v1.6.0
* Update docs
diff --git a/docs/dyn/sheets_v4.spreadsheets.sheets.html b/docs/dyn/sheets_v4.spreadsheets.sheets.html
index 871f6a4..885d5cb 100644
--- a/docs/dyn/sheets_v4.spreadsheets.sheets.html
+++ b/docs/dyn/sheets_v4.spreadsheets.sheets.html
@@ -107,7 +107,13 @@
     "index": 42, # The index of the sheet within the spreadsheet.
         # When adding or updating sheet properties, if this field
         # is excluded then the sheet will be added or moved to the end
-        # of the sheet list.
+        # of the sheet list. When updating sheet indices or inserting
+        # sheets, movement is considered in "before the move" indexes.
+        # For example, if there were 3 sheets (S1, S2, S3) in order to
+        # move S1 ahead of S2 the index would have to be set to 2. A sheet
+        # index update request will be ignored if the requested index is
+        # identical to the sheets current index or if the requested new
+        # index is equal to the current sheet index + 1.
     "title": "A String", # The name of the sheet.
     "gridProperties": { # Properties of a grid. # Additional properties of the sheet if this sheet is a grid.
         # (If the sheet is an object sheet, containing a chart or image, then