Document the null register

README.asciidoc

@@ -975,6 +975,7 @@ contain some special data:
  * `%`: current buffer name
  * `.`: current selection contents
  * `#`: selection indices (first selection has 1, second has 2, ...)
+ * `_`: null register, always empty
 
 Default registers
 ^^^^^^^^^^^^^^^^^

doc/manpages/registers.asciidoc

@@ -56,6 +56,9 @@ contain some special data
 *#*::
 	selection indices (first selection has 1, second has 2, ...)
 
+*_*::
+	null register, always empty
+
 Integer registers
 -----------------
 Registers *1* to *9* hold the grouped sub-matches of the regular

src/input_handler.cc

@@ -145,6 +145,7 @@ constexpr StringView register_doc =
     "    * .: selection contents\n"
     "    * #: selection index\n"
     "    * [0-9]: selections capture group\n"
+    "    * _: null register\n"
     "    * \": default yank/paste register\n"
     "    * @: default macro register\n"
     "    * /: default search register\n"