keyboardBegin.adoc: Add Notes and Warnings

Move the note on custom layouts to this new section, and be more
specific on the location of the KeyboardLayout.h file. As per per1234's
suggestions.

Author: edgar-bonet

Language/Functions/USB/Keyboard/keyboardBegin.adoc

@@ -39,8 +39,6 @@ Currently, the library supports the following national keyboard layouts:
 * `KeyboardLayout_fr_FR`: France
 * `KeyboardLayout_it_IT`: Italy
 
-Custom layouts can be created by copying and modifying an existing layout. See the instructions in the file KeyboardLayout.h.
-
 
 [float]
 === Returns
@@ -82,5 +80,10 @@ void loop() {
 }
 ----
 
+
+[float]
+=== Notes and Warnings
+Custom layouts can be created by copying and modifying an existing layout. See the instructions in the Keyboard library's KeyboardLayout.h file.
+
 --
 // HOW TO USE SECTION ENDS