Better documentation for 'multires' expressions

Manual has a new section explaining multires expressions, lists of
expressions, and adjustments. This commit also corrects some comments
in the code.

lauxlib.c

@@ -526,7 +526,8 @@ static void newbox (lua_State *L) {
 
 /*
 ** Compute new size for buffer 'B', enough to accommodate extra 'sz'
-** bytes.
+** bytes. (The test for "double is not big enough" also gets the
+** case when the multiplication by 2 overflows.)
 */
 static size_t newbuffsize (luaL_Buffer *B, size_t sz) {
   size_t newsize = B->size * 2;  /* double buffer size */
@@ -611,7 +612,7 @@ LUALIB_API void luaL_pushresultsize (luaL_Buffer *B, size_t sz) {
 ** box (if existent) is not on the top of the stack. So, instead of
 ** calling 'luaL_addlstring', it replicates the code using -2 as the
 ** last argument to 'prepbuffsize', signaling that the box is (or will
-** be) bellow the string being added to the buffer. (Box creation can
+** be) below the string being added to the buffer. (Box creation can
 ** trigger an emergency GC, so we should not remove the string from the
 ** stack before we have the space guaranteed.)
 */