Document APIs relating to built-in and custom extensions

While I'm here, unexport STACK_OF(X509V3_EXT_METHOD). We use it
internally, but it never appears in any public APIs, and there's no real
reason for any caller to use it.

Bug: 426
Change-Id: I6057834847a37f435d1b687701a3e65b5afb2890
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/66387
Auto-Submit: David Benjamin <davidben@google.com>
Commit-Queue: Bob Beck <bbe@google.com>
Reviewed-by: Bob Beck <bbe@google.com>

crypto/x509/v3_lib.c

@@ -70,6 +70,8 @@
 
 #include "ext_dat.h"
 
+DEFINE_STACK_OF(X509V3_EXT_METHOD)
+
 static STACK_OF(X509V3_EXT_METHOD) *ext_list = NULL;
 
 static int ext_stack_cmp(const X509V3_EXT_METHOD *const *a,

include/openssl/obj.h

@@ -194,8 +194,8 @@ OPENSSL_EXPORT int OBJ_obj2txt(char *out, int out_len, const ASN1_OBJECT *obj,
 // duplicate OIDs, short names, or long names. If two callers in the same
 // address space add conflicting values, only one registration will take effect.
 // Avoid this function if possible. Instead, callers can process OIDs unknown to
-// BoringSSL by acting on the byte representation directly. See |OBJ_get0_data|
-// and |OBJ_length|.
+// BoringSSL by acting on the byte representation directly. See
+// |ASN1_OBJECT_create|, |OBJ_get0_data|, and |OBJ_length|.
 OPENSSL_EXPORT int OBJ_create(const char *oid, const char *short_name,
                               const char *long_name);