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/eval.c

@@ -345,13 +345,6 @@ typedef enum {
   VAR_FLAVOUR_VIMINFO           /* all uppercase */
 } var_flavour_T;
 
-/*
- * Array to hold the value of v: variables.
- * The value is in a dictitem, so that it can also be used in the v: scope.
- * The reason to use this table anyway is for very quick access to the
- * variables with the VV_ defines.
- */
-
 /* values for vv_flags: */
 #define VV_COMPAT       1       /* compatible, also used without "v:" */
 #define VV_RO           2       /* read-only */
@@ -359,6 +352,10 @@ typedef enum {
 
 #define VV_NAME(s, t)   s, {{t, 0, {0}}, 0, {0}}, {0}
 
+// Array to hold the value of v: variables.
+// The value is in a dictitem, so that it can also be used in the v: scope.
+// The reason to use this table anyway is for very quick access to the
+// variables with the VV_ defines.
 static struct vimvar {
   char        *vv_name;         /* name of variable, without v: */
   dictitem_T vv_di;             /* value and name for key */

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

src/nvim/msgpack_rpc/channel.c

@@ -723,7 +723,7 @@ static void complete_call(msgpack_object *obj, Channel *channel)
 
 static void call_set_error(Channel *channel, char *msg)
 {
-  ELOG("Msgpack-RPC error: %s", msg);
+  ELOG("msgpack-rpc: %s", msg);
   for (size_t i = 0; i < kv_size(channel->call_stack); i++) {
     ChannelCallFrame *frame = kv_A(channel->call_stack, i);
     frame->returned = true;