bug#62687: 30.0.50; Eglot (eglot--sig-info): SignatureInformation's Documentation is never shown when it's of type MarkupContent

[Mekeor Melire]

What's the problem?

When you enable eglot-mode from Eglot 1.14; when it then 
successfully
connects to a language server; when you then move the cursor to
"something callable", Eglot is supposed to show the Documentation 
from
the SignatureInformation, as specified in the language server 
protocol.
This SignatureInformation's Documentation can either be of type 
string,
or of type MarkupContent [1]. Eglot will never show it, if it's of
latter type.

Why does this happen?

You enable eglot-mode. Eglot successfully connects to a language 
server.
Eglot enables eldoc-mode. When you move the cursor, the function
`eglot-signature-eldoc-function' is called because it's a member 
of
`eldoc-documentation-functions'. This function,
`eglot-signature-eldoc-function`, calls `eglot--sig-info'. That 
function
will only show the documentation if it's of type string [2].

--8<---------------cut here---------------start------------->8---
       ;; Decide whether to add one-line-summary to signature 
       line
       (when (and (stringp documentation)
                  (string-match "[[:space:]]*\\([^.\r\n]+[.]?\\)"
                                documentation))
         (setq documentation (match-string 1 documentation))
         (unless (string-prefix-p (string-trim documentation) 
         label)
           (goto-char (point-max))
           (insert ": " (eglot--format-markup documentation))))
--8<---------------cut here---------------end--------------->8---


How to see this happen?

   Install GNU Emacs from master-branch; use any commit after 
   "Eglot:
   Bump to 1.14" 8125d4cfc5605ead9102b7d823c4241029eb76cc. You 
   might
   need to build with tree-sitter and install a tree-sitter 
   grammar for
   typescript, so that you can use `typescript-ts-mode' because 
   Emacs
   has no other mode for typescript.

   Install Git and Node.js, including NPM.

   Install typescript-language-server:
       npm install --global typescript-language-server

   Clone the minimal-reproducing-example:
       git clone 
       https://github.com/mekeor/emacs-eglot-minimal-reproducing-example
       cd emacs-eglot-minimal-reproducing-example

   Install the project dependencies:
       npm install

   Open source file with Emacs:
       emacs ./main.ts

   In Emacs, type:
       M-x typescript-ts-mode RET
       M-x eglot RET
       C-3 C-5 C-2 C-f

   In the echo-area, you will see the type signature of the 
   function
   "fastify.addHook", but there will be no documentation:

2023-04-06-000337_496x610_scrot.png
Description: PNG image

[1] 
https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#signatureInformation

[2] 
https://git.savannah.gnu.org/cgit/emacs.git/tree/lisp/progmodes/eglot.el?id=fa669c4b17c04eff852eb23a6179ccb8fab864db#n3132