Fix joaotavora#272: also use signature label offsets for parameter info

According to the LSP specification, a parameter of a callable-signature
has a label and a optional doc-commet. The label of a parameter
information is either a string or an inclusive start and exclusive end
offsets within its containing signature label.

Previously, this was only taken in account for highlighting the
parameter in the definition signature.

* eglot.el (eglot--sig-info): Handle signature label offsets
  when printing the signature parameter information.

Copyright-paperwork-exempt: yes

eglot.el

@@ -2055,7 +2055,10 @@ is not active."
                  (goto-char (point-max))
                  (insert "\n"
                          (propertize
-                          label 'face 'eldoc-highlight-function-argument)
+                          (if (stringp label)
+                              label
+                            (apply #'buffer-substring (mapcar #'1+ label)))
+                          'face 'eldoc-highlight-function-argument)
                          ": " (eglot--format-markup documentation))))))
          (buffer-string))))
    when moresigs concat "\n"))