Eglot: simplify eglot--sig-info and fix edge cases (bug#62687)

Eglot: simplify eglot--sig-info and fix edge cases (bug#62687)

The documentation of each signature was rarely rendered because of
obsolete logic that skipped MarkupContent objects.

The new function follows the principle that echo are gets as little as
possible, while the *eldoc* buffer gets as much as possible (except,
for the individual parameter documentation, as I couldn't find a
single server that uses it).

Tested with clangd, pylsp, jdtls, gopls, rust-analyzer, zls.

* lisp/progmodes/eglot.el (eglot--sig-info): Simplify.