Update generic.py

[bjiro]

Added documentation on head and tail methods. Parameter n can be larger than the object size.

• closes Backport PR #46069 on branch 1.4.x (DOC: fix ref in 1.4.2 release notes) #46073

[pep8speaks]

Hello @benjamrio! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file pandas/core/generic.py:

Line 5164:1: W293 blank line contains whitespace
Line 5165:89: E501 line too long (97 > 88 characters)
Line 5170:38: W291 trailing whitespace
Line 5240:89: E501 line too long (97 > 88 characters)
Line 5241:1: W293 blank line contains whitespace
Line 5245:38: W291 trailing whitespace