Add proper handling of \note, \warning and \retval command + change render kind of command arguments.

[tcottin]

PR for some followup issues from clangd/clangd#529.

\note and \warning:
In the hover card, they are now displayed with heading and enclosing rulers.

\retval commands:
Each \retval command is now a bullet point under the return section of the hover card.

[Stehsaer]

It's working great as intended!

Can other commands like @details be added the same way we add paragraphs for @note and @warning?

Update: Yes we can, just add some more changes as given below. This should work well for "generic" ones that only needs a Header and its content to be shown. Compiled and tested it out, working quite well.

diff --git a/clang-tools-extra/clangd/SymbolDocumentation.cpp b/clang-tools-extra/clangd/SymbolDocumentation.cpp
index 86bd6162b..1638e4b55 100644
--- a/clang-tools-extra/clangd/SymbolDocumentation.cpp
+++ b/clang-tools-extra/clangd/SymbolDocumentation.cpp
@@ -216,6 +216,7 @@ public:
       break;
     case comments::CommandTraits::KCI_note:
     case comments::CommandTraits::KCI_warning:
+    case comments::CommandTraits::KCI_details:
       commandToHeadedParagraph(B);
       break;
     case comments::CommandTraits::KCI_retval: {