Document failure conditions of some EVP, HMAC, and CBB functions

- Document failure conditions of EVP_MD_CTX_copy_ex,
  EVP_DigestInit_ex, HMAC_Init_ex, and CBB_init

Change-Id: I643d1b92e88e7f690fa555f7d908317a23e5cd95
Reviewed-on: https://boringssl-review.googlesource.com/30964
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

include/openssl/bytestring.h

diff --git a/include/openssl/bytestring.h b/include/openssl/bytestring.h
index 6ed1644..3057604 100644
--- a/include/openssl/bytestring.h
+++ b/include/openssl/bytestring.h
@@ -340,7 +340,7 @@
 
 // CBB_init initialises |cbb| with |initial_capacity|. Since a |CBB| grows as
 // needed, the |initial_capacity| is just a hint. It returns one on success or
-// zero on error.
+// zero on allocation failure.
 OPENSSL_EXPORT int CBB_init(CBB *cbb, size_t initial_capacity);
 
 // CBB_init_fixed initialises |cbb| to write to |len| bytes at |buf|. Since

include/openssl/digest.h

diff --git a/include/openssl/digest.h b/include/openssl/digest.h
index 4077d90..4a2b710 100644
--- a/include/openssl/digest.h
+++ b/include/openssl/digest.h
@@ -119,7 +119,7 @@
 OPENSSL_EXPORT void EVP_MD_CTX_free(EVP_MD_CTX *ctx);
 
 // EVP_MD_CTX_copy_ex sets |out|, which must already be initialised, to be a
-// copy of |in|. It returns one on success and zero on error.
+// copy of |in|. It returns one on success and zero on allocation failure.
 OPENSSL_EXPORT int EVP_MD_CTX_copy_ex(EVP_MD_CTX *out, const EVP_MD_CTX *in);
 
 // EVP_MD_CTX_reset calls |EVP_MD_CTX_cleanup| followed by |EVP_MD_CTX_init|. It
@@ -131,7 +131,7 @@
 
 // EVP_DigestInit_ex configures |ctx|, which must already have been
 // initialised, for a fresh hashing operation using |type|. It returns one on
-// success and zero otherwise.
+// success and zero on allocation failure.
 OPENSSL_EXPORT int EVP_DigestInit_ex(EVP_MD_CTX *ctx, const EVP_MD *type,
                                      ENGINE *engine);
 

include/openssl/hmac.h

diff --git a/include/openssl/hmac.h b/include/openssl/hmac.h
index 8491b8d..20f3e4d 100644
--- a/include/openssl/hmac.h
+++ b/include/openssl/hmac.h
@@ -105,7 +105,7 @@
 // function and |key| as the key. For a non-initial call, |md| may be NULL, in
 // which case the previous hash function will be used. If the hash function has
 // not changed and |key| is NULL, |ctx| reuses the previous key. It returns one
-// on success or zero otherwise.
+// on success or zero on allocation failure.
 //
 // WARNING: NULL and empty keys are ambiguous on non-initial calls. Passing NULL
 // |key| but repeating the previous |md| reuses the previous key rather than the