Move structs to a 'Private structures' section in ssl.h.

To be consistent with some of the other headers and because SSL_METHOD
no longer has a place to anchor documentation, move the type
documentation up to the corresponding section headers, rather than
attached to a convenient function.

Also document thread-safety properties of SSL and SSL_CTX.

Change-Id: I7109d704d28dda3f5d83c72d86fe31bc302b816e
Reviewed-on: https://boringssl-review.googlesource.com/5876
Reviewed-by: Adam Langley <agl@google.com>

include/openssl/base.h

@@ -231,8 +231,10 @@ typedef struct rsa_st RSA;
 typedef struct sha256_state_st SHA256_CTX;
 typedef struct sha512_state_st SHA512_CTX;
 typedef struct sha_state_st SHA_CTX;
+typedef struct ssl_cipher_st SSL_CIPHER;
 typedef struct ssl_ctx_st SSL_CTX;
 typedef struct ssl_custom_extension SSL_CUSTOM_EXTENSION;
+typedef struct ssl_method_st SSL_METHOD;
 typedef struct ssl_st SSL;
 typedef struct st_ERR_FNS ERR_FNS;
 typedef struct v3_ext_ctx X509V3_CTX;