item key helper has outdated descriptions, some item keys are completely missing and there are other issues.

these strings should be changed as little as possible to reduce workload for translators. additionally, it would be bad to have the descriptions different from the manual as double effort would mean one of them becoming outdated (as evident by the current situation in the frontend).

it has been decided to sync its contents from the manual, including "description" and "return value" columns only, otherwise the content would be too long.

ZBXNEXT-2668 deals with providing even more information in the key helper

note that this issue deals with agent items only, other item types to be handled in separate issues. note that all manual pages should be pedantically cleaned up before syncing them to reduce changes in translated strings (like it was done in ZBX-8811)