breakdown_factor type in singlediode.bishop88 docs

[reepoi]

Describe the bug
The docs of the singlediode.bishop88* functions say that breakdown_factor can be numeric, but these functions crash when breakdown_factor is an array of length two or more. This is caused by the if-statement here.

Expected behavior
The functions can be modified to support the numeric type for breakdown_factor, or the docs can be updated to say that breakdown_factor must be a float.

Versions:

• pvlib.__version__: 0.10.1