COMPAT: Standardize method signatures
Currently, the parameter for values that one needs to insert into an array-like object such as `DataFrame` or `Series` is inconsistent across implementations.  For example, `base.py` surfaces it as `key`, whereas `series.py` surfaces it as `v`.

This issue was identified while fixing an issue for `v0.18.1` (xref #12413), but in the interests of maintaining backwards compatibility for a minor release, this change should be made as early as `v0.19.0` with the following API comment:

"The signature for `searchsorted` has changed. It is now `searchsorted(self, value, side='left', sorter=None)`."

List of functions with signature inconsistencies:
`searchsorted`
`repeat`

@jreback 
@jorisvandenbossche 
