gsm48: gsm48_decode_freq_list: Improve documentation of paramter f

Change-Id: Id8e3afff737211ded3c5689f2c83b1a544b42f9d
diff --git a/src/gsm/gsm48_ie.c b/src/gsm/gsm48_ie.c
index 1baf287..ffe3eba 100644
--- a/src/gsm/gsm48_ie.c
+++ b/src/gsm/gsm48_ie.c
@@ -807,7 +807,7 @@
 }
 
 /*! Decode TS 04.08 Cell Channel Description IE (10.5.2.1b) and other frequency lists
- *  \param[out] f Caller-provided output memory
+ *  \param[out] f Caller-provided output memory, an array of 1024 elements
  *  \param[in] cd Cell Channel Description IE
  *  \param[in] len Length of \a cd in bytes
  *  \returns 0 on success; negative on error */