Add documentation notes about the required size of the signature buffers
diff --git a/include/mbedtls/rsa.h b/include/mbedtls/rsa.h
index 906c427..cba692b 100644
--- a/include/mbedtls/rsa.h
+++ b/include/mbedtls/rsa.h
@@ -904,7 +904,8 @@
* the size of the hash corresponding to \p md_alg.
* \param sig The buffer to hold the signature. This must be a writable
* buffer of length \c ctx->len Bytes. For example, \c 256 Bytes
- * for an 2048-bit RSA modulus.
+ * for an 2048-bit RSA modulus. A buffer length of
+ * #MBEDTLS_MPI_MAX_SIZE is always safe.
*
* \return \c 0 if the signing operation was successful.
* \return An \c MBEDTLS_ERR_RSA_XXX error code on failure.
@@ -951,7 +952,8 @@
* the size of the hash corresponding to \p md_alg.
* \param sig The buffer to hold the signature. This must be a writable
* buffer of length \c ctx->len Bytes. For example, \c 256 Bytes
- * for an 2048-bit RSA modulus.
+ * for an 2048-bit RSA modulus. A buffer length of
+ * #MBEDTLS_MPI_MAX_SIZE is always safe.
*
* \return \c 0 if the signing operation was successful.
* \return An \c MBEDTLS_ERR_RSA_XXX error code on failure.
@@ -1012,7 +1014,8 @@
* the size of the hash corresponding to \p md_alg.
* \param sig The buffer to hold the signature. This must be a writable
* buffer of length \c ctx->len Bytes. For example, \c 256 Bytes
- * for an 2048-bit RSA modulus.
+ * for an 2048-bit RSA modulus. A buffer length of
+ * #MBEDTLS_MPI_MAX_SIZE is always safe.
*
* \return \c 0 if the signing operation was successful.
* \return An \c MBEDTLS_ERR_RSA_XXX error code on failure.