fix(lsp): SignatureHelp docstring is not escaped #16702

Problem:
Nvim LSP client always treats signature.documentation as markdown, even
if the server returns a plain string.
Per https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#signatureInformation
in a SignatureInformation response, the documentation field can be
either "string" or "MarkupContent".

Solution:
If signature.documentation is a string, treat it as "plaintext".

Closes #16563

runtime/lua/vim/lsp/util.lua

@@ -1002,6 +1002,12 @@ function M.convert_signature_help_to_markdown_lines(signature_help, ft, triggers
   end
   list_extend(contents, split(label, '\n', { plain = true }))
   if signature.documentation then
+    -- if LSP returns plain string, we treat it as plaintext. This avoids
+    -- special characters like underscore or similar from being interpreted
+    -- as markdown font modifiers
+    if type(signature.documentation) == 'string' then
+      signature.documentation = { kind = 'plaintext', value = signature.documentation }
+    end
     M.convert_input_to_markdown_lines(signature.documentation, contents)
   end
   if signature.parameters and #signature.parameters > 0 then