Eglot: respect user's Eldoc configuration by default

This change addresses the problems reported in many Elglot reports
dating back to early 2021 at least:

  joaotavora/eglot#648
  joaotavora/eglot#894
  joaotavora/eglot#920
  joaotavora/eglot#1031
  joaotavora/eglot#1171

In one form or another, the reports point out that the multiple pieces
of information about the "thing at point" made available by the LSP
server are not all being considered by the ElDoc system.

The reason for this is Eglot setting/trampling the variables
'eldoc-documentation-strategy' and 'eldoc-documentation-functions' in
its minor more entry function.

The reason it did that is historical and is partially described in the
issues above.  But, evidently, it never made much sense, because so
many people want to override it, which requires setting
'eldoc-documentation-strategy' to the non-default value
'eldoc-documentation-compose'.

The problem was made worse by the fact that setting it as usual in
either the Customize menu or their init file didn't work, requiring a
fairly complex Elisp snippet.  That is now solved as of this commit.

If the user does not do any setting, then Eglot works basically the
same as before (i.e. shows only one piece of information).

It is arguable that the default value for
'eldoc-documentation-strategy' should change globally to
'eldoc-documentation-compose', but that has other subtle implications
and is not part of this commit.

* lisp/progmodes/eglot.el (eglot--managed-mode): Don't set Eldoc
variables greedily.

Author: joaotavora

lisp/progmodes/eglot.el

@@ -1760,11 +1760,6 @@ Use `eglot-managed-p' to determine if current buffer is managed.")
     (add-hook 'change-major-mode-hook #'eglot--managed-mode-off nil t)
     (add-hook 'post-self-insert-hook 'eglot--post-self-insert-hook nil t)
     (add-hook 'pre-command-hook 'eglot--pre-command-hook nil t)
-    (eglot--setq-saving eldoc-documentation-functions
-                        '(eglot-signature-eldoc-function
-                          eglot-hover-eldoc-function))
-    (eglot--setq-saving eldoc-documentation-strategy
-                        #'eldoc-documentation-enthusiast)
     (eglot--setq-saving xref-prompt-for-identifier nil)
     (eglot--setq-saving flymake-diagnostic-functions '(eglot-flymake-backend))
     (eglot--setq-saving company-backends '(company-capf))
@@ -1773,7 +1768,12 @@ Use `eglot-managed-p' to determine if current buffer is managed.")
       (add-function :before-until (local 'imenu-create-index-function)
                     #'eglot-imenu))
     (unless (eglot--stay-out-of-p 'flymake) (flymake-mode 1))
-    (unless (eglot--stay-out-of-p 'eldoc) (eldoc-mode 1))
+    (unless (eglot--stay-out-of-p 'eldoc)
+      (add-hook 'eldoc-documentation-functions #'eglot-hover-eldoc-function
+                nil t)
+      (add-hook 'eldoc-documentation-functions #'eglot-signature-eldoc-function
+                nil t)
+      (eldoc-mode 1))
     (cl-pushnew (current-buffer) (eglot--managed-buffers (eglot-current-server))))
    (t
     (remove-hook 'after-change-functions 'eglot--after-change t)