Document non-GNU IConv on macOS

Author: Flamefire

doc/building_boost_locale.txt

@@ -32,7 +32,8 @@
     so you need to either use GNU iconv or link with the ICU library.
 -   If the iconv library is not found on Darwin/Mac OS X builds make sure there
     are not multiple iconv installations and provide the -sICONV_PATH build option
-    to point to the correct location of the iconv library.
+    to point to the correct location of the iconv library.  
+    Using the HomeBrew installed GNU IConv is highly recommended!
 
 \subsection bb_building_proc Building Process
 

doc/main.txt

@@ -54,6 +54,13 @@ It is based on the operating system native API or on the standard
 C++ library support. Sacrificing some less important features,
 Boost.Locale becomes less powerful but lighter and easier to deploy.
 
+Charset conversion is also provided through the lightweight IConv library.
+When that is not available ICU or operating system native APIs are used.
+
+\warning The system IConv library on Apple macOS may not be GNU conformant
+and can lead to unexpected results during encoding conversions.
+Using the GNU IConv provided by HomeBrew is highly recommended.
+
 
 \section main_tutorial Tutorials