core: document keybind actions better

The current documentation for actions are very sparse and would leave
someone (even including contributors) as to what exactly they do.
On top of that there are many stylistic and grammatical problems that are
simply no longer in line with our current standards, and certainly not
on par with our configuration options reference.

Hence, I've taken it upon myself to add, clarify, supplement, edit and
even rewrite the documentation for most of these actions, in a wider
effort of trying to offer better, clearer documentation for our users.

src/input/command.zig

@@ -119,7 +119,7 @@ fn actionCommands(action: Action.Key) []const Command {
         .paste_from_clipboard => comptime &.{.{
             .action = .paste_from_clipboard,
             .title = "Paste from Clipboard",
-            .description = "Paste the contents of the clipboard.",
+            .description = "Paste the contents of the main clipboard.",
         }},
 
         .paste_from_selection => comptime &.{.{