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Introduce eldoc-documetation-strategy

This is just a new name for the obsolete eldoc-documentation-function.
Four different doc display strategies are provided.

* lisp/emacs-lisp/eldoc.el (eldoc-highlight-function-argument)
(eldoc-argument-case, global-eldoc-mode, turn-on-eldoc-mode)
(eldoc-documentation-functions): Mention
eldoc-documentation-strategy.
(eldoc--make-callback): Rework.
(eldoc-documentation-default, eldoc-documentation-compose):
Mention eldoc-documentation-strategy.
(eldoc-documentation-compose-eagerly): New function.
(eldoc-documentation-function): Now an obsolete var alias.
(eldoc-documentation-strategy): Rename from
eldoc-documentation-function.
(eldoc--supported-p): Use eldoc-documentation-strategy
(eldoc-print-current-symbol-info): Rework with cl-labels.