Add docstring for undocumented gradient method

[stillyslalom]

gradient(F,x::Vector) is useful, but not included in the manual, making method discovery difficult. Also replaced h with x in function body (since h conventionally denotes inter-point spacing rather than location).

[andreasnoack]

See also #16113

[stillyslalom]

Ah, it's part of the battery-removal project. I'm still confused as to why convenience functions are being moved from base to myriads of hard-to-discover packages without some sort of stdlib mechanism in place. I understand the maintainers' desire to trim down base, but I wish they'd slow down on migrating convenience functions out until there's a good way for new users to access basic functions without having to dig through unevenly-documented external packages.

[fredrikekre]

#23816