doc: gcc handles duplicate literal strings (#7531)

Author: d-a-v

doc/reference.rst

@@ -247,12 +247,17 @@ Progmem
 
 The Program memory features work much the same way as on a regular
 Arduino; placing read only data and strings in read only memory and
-freeing heap for your application. The important difference is that on
-the ESP8266 the literal strings are not pooled. This means that the same
-literal string defined inside a ``F("")`` and/or ``PSTR("")`` will take
-up space for each instance in the code. So you will need to manage the
+freeing heap for your application.
+
+In core versions prior to 2.7, the important difference is that on the
+ESP8266 the literal strings are not pooled.  This means that the same
+literal string defined inside a ``F("")`` and/or ``PSTR("")`` will take up
+space for each instance in the code.  So you will need to manage the
 duplicate strings yourself.
 
+Starting from v2.7, this is no longer true: duplicate literal strings within
+r/o memory are now handled.
+
 There is one additional helper macro to make it easier to pass
 ``const PROGMEM`` strings to methods that take a ``__FlashStringHelper``
 called ``FPSTR()``. The use of this will help make it easier to pool