Document alternative functions to BIO_f_base64.

Also point out that we're a cryptography library, not a text encoding library.
Not that that'll dissuade anyone.

Change-Id: Ia324e08c5cdd108fa182d2610f80447262e0bd5c
Reviewed-on: https://boringssl-review.googlesource.com/31664
Reviewed-by: Adam Langley <agl@google.com>

include/openssl/base64.h

@@ -67,7 +67,10 @@ extern "C" {
 // base64 functions.
 //
 // For historical reasons, these functions have the EVP_ prefix but just do
-// base64 encoding and decoding.
+// base64 encoding and decoding. Note that BoringSSL is a cryptography library,
+// so these functions are implemented with side channel protections, at a
+// performance cost. For other base64 uses, use a general-purpose base64
+// implementation.
 
 
 // Encoding

include/openssl/bio.h

@@ -729,6 +729,8 @@ OPENSSL_EXPORT int BIO_get_init(BIO *bio);
 // |BIO_flush| when done writing, to signal that no more data are to be
 // encoded. The flag |BIO_FLAGS_BASE64_NO_NL| may be set to encode all the data
 // on one line.
+//
+// Use |EVP_EncodeBlock| and |EVP_DecodeBase64| instead.
 OPENSSL_EXPORT const BIO_METHOD *BIO_f_base64(void);
 
 OPENSSL_EXPORT void BIO_set_retry_special(BIO *bio);