docs/manual: indicate how to handle BR2_TOOLCHAIN_HAS_THREADS_NPTL

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/manual/adding-packages-directory.txt

@@ -235,7 +235,12 @@ use in the comment.
 
 * thread support
 ** Dependency symbol: +BR2_TOOLCHAIN_HAS_THREADS+
-** Comment string: +threads+
+** Comment string: +threads+ (unless +BR2_TOOLCHAIN_HAS_THREADS_NPTL+
+   is also needed, in which case, specifying only +NPTL+ is sufficient)
+
+* NPTL thread support
+** Dependency symbol: +BR2_TOOLCHAIN_HAS_THREADS_NPTL+
+** Comment string: +NPTL+
 
 * RPC support
 ** Dependency symbol: +BR2_TOOLCHAIN_HAS_NATIVE_RPC+