Update GCM buffer overlap documentation

This commit updates GCM buffer overlap documentation to explicitly
support overlapping encryption/decryption buffers for GCM.

Signed-off-by: Harry Ramsey <harry.ramsey@arm.com>

include/mbedtls/gcm.h

@@ -107,10 +107,9 @@ int mbedtls_gcm_setkey(mbedtls_gcm_context *ctx,
 /**
  * \brief           This function performs GCM encryption or decryption of a buffer.
  *
- * \note            For encryption, the output buffer can be the same as the
+ * \note            The output buffer \p output can be the same as the input
- *                  input buffer. For decryption, the output buffer cannot be
+ *                  buffer \p input. If \p output is greater than \p input, they
- *                  the same as input buffer. If the buffers overlap, the output
+ *                  cannot overlap.
- *                  buffer must trail at least 8 Bytes behind the input buffer.
  *
  * \warning         When this function performs a decryption, it outputs the
  *                  authentication tag and does not verify that the data is
@@ -243,9 +242,11 @@ int mbedtls_gcm_starts(mbedtls_gcm_context *ctx,
  *                  Bytes. Only the last call before calling
  *                  mbedtls_gcm_finish() can be less than 16 Bytes.
  *
- * \note            For decryption, the output buffer cannot be the same as
+ * \note            The output buffer \p output can be the same as the input
- *                  input buffer. If the buffers overlap, the output buffer
+ *                  buffer \p input. If \p output is greater than \p input, they
- *                  must trail at least 8 Bytes behind the input buffer.
+ *                  cannot overlap. Implementations which require
+ *                  MBEDTLS_GCM_ALT //no-check-names to be enabled may not
+ *                  provide support for overlapping buffers. *
  *
  * \param ctx       The GCM context. This must be initialized.
  * \param length    The length of the input data. This must be a multiple of