clarify that CborIndefiniteLength creates an indefinite map, which is not always supported

diff --git a/src/cborencoder.c b/src/cborencoder.c
index e21e83d..3c24188 100644
--- a/src/cborencoder.c
+++ b/src/cborencoder.c

@@ -510,7 +510,7 @@
* The number of pair of items inserted into the map must be exactly \a length
* items, otherwise the stream is invalid. If the number is not known
* when creating the map, the constant \ref CborIndefiniteLength may be passed as