Switch default merge_type for `Worksheet.merge_cells`. Add link to Sheets API in the method’s docstring. 
Switch default merge_type for `Worksheet.merge_cells`.
Add link to Sheets API in the method’s docstring. 

Rationale: although no default MergeType is 
specified in Sheets API, the default behaviour of 
the “Merge cells” button in the Sheets UI 
is “Merge all”.
burnash/gspread