doc: Misc. cleanup

Add missing parentheses and whatnot, move dangling comment, etc. Some
specific items worth mentioning:

Fixed some references to non-existent tags, found via `make html`

msgpack_rpc/channel.c:
  ELOG already prefixes each line with "error @ ..."

src/nvim/main.c

@@ -1961,7 +1961,7 @@ static void mainerr(int n, const char *str)
 }
 
 
-/// Prints help message and exits; used for 'nvim -h' & 'nvim --help'
+/// Prints help message for "nvim -h" or "nvim --help" and exits.
 static void usage(void)
 {
   signal_stop();              // kill us with CTRL-C here, if you like