err.h: Add missing kerneldocs for error pointer functions

Add kerneldocs for ERR_PTR(), PTR_ERR(), PTR_ERR_OR_ZERO(), IS_ERR(),
and IS_ERR_OR_NULL(). Doing so will help convert hundreds of mentions
of them in existing documentation into automatic cross-references.

Also add kerneldocs for IS_ERR_VALUE(). Doing so adds no automatic
cross-references, but this macro has a slightly different use case
than the functionally similar IS_ERR(), and documenting it may be
helpful to readers who encounter it in existing code.

ERR_CAST() already has kerneldocs and has not been touched.

Signed-off-by: James Seo <[email protected]>
Signed-off-by: Jonathan Corbet <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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