USE_TLS_SRP will be true if *any* selected TLS backend can use SRP
HAVE_OPENSSL_SRP is defined when OpenSSL can use it
HAVE_GNUTLS_SRP is defined when GnuTLS can use it
Clarify in the curl_verison_info docs that CURL_VERSION_TLSAUTH_SRP is
set if at least one of the supported backends offers SRP.
Reported-by: Stefan Strogin
Fixes#5865Closes#5870