Document that BIO flags should usually be left alone.

Some BIO flags can be memory safety risks when misused; others simply
are not meant to be used after BIO creation. However, outright blocking
is not an option, as users can define their own BIOs with their own flag
semantics.

Change-Id: Iad1a3dfe942155626da2163c07909f1a6a6a6964
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/91007
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
Auto-Submit: Rudolf Polzer <rpolzer@google.com>

Author: divVerent

include/openssl/bio.h

@@ -131,10 +131,13 @@ OPENSSL_EXPORT int BIO_reset(BIO *bio);
 // case of BIO_pair this always returns non-zero.
 OPENSSL_EXPORT int BIO_eof(BIO *bio);
 
-// BIO_set_flags ORs |flags| with |bio->flags|.
+// BIO_set_flags ORs |flags| with |bio->flags|. Unless otherwise documented,
+// flags are private to either BoringSSL or the custom |BIO_METHOD|.
 OPENSSL_EXPORT void BIO_set_flags(BIO *bio, int flags);
 
 // BIO_clear_flags ANDs |bio->flags| with the bitwise-complement of |flags|.
+// Unless otherwise documented, flags are private to either BoringSSL or the
+// custom |BIO_METHOD|.
 OPENSSL_EXPORT void BIO_clear_flags(BIO *bio, int flags);
 
 // BIO_test_flags returns |bio->flags| AND |flags|.