docs: replace xml-style markup with markdown in comments

Replace XML-style markup tags in comments with markdown equivalents:
- <code>...</code> to `...` (inline code)
- <tt>...</tt> to `...` (teletype/monospace)
- <i>...</i> to *...* (italics/emphasis)
- +...+ to `...` (parameter/variable references)

Updated 80+ files across core source, headers, mrbgems, and libraries
to use consistent markdown formatting in documentation comments.
Handled edge cases including special characters like <=> operators.

Co-authored-by: Atlassian Rovo Dev

src/variable.c

@@ -689,7 +689,7 @@ cv_i(mrb_state *mrb, mrb_sym sym, mrb_value v, void *p)
  *  call-seq:
  *     mod.class_variables(inherit=true)   -> array
  *
- *  Returns an array of the names of class variables in <i>mod</i>.
+ *  Returns an array of the names of class variables in *mod*.
  *
  *     class One
  *       @@var1 = 1