cURL header file indicates the following feature testing constants, of which only 4 are listed at http://www.php.net/curl.constants

#define CURL_VERSION_IPV6         (1<<0)  /* IPv6-enabled */
#define CURL_VERSION_KERBEROS4    (1<<1)  /* Kerberos V4 auth is supported
                                             (deprecated) */
#define CURL_VERSION_SSL          (1<<2)  /* SSL options are present */
#define CURL_VERSION_LIBZ         (1<<3)  /* libz features are present */
#define CURL_VERSION_NTLM         (1<<4)  /* NTLM auth is supported */
#define CURL_VERSION_GSSNEGOTIATE (1<<5)  /* Negotiate auth is supported
                                             (deprecated) */
#define CURL_VERSION_DEBUG        (1<<6)  /* Built with debug capabilities */
#define CURL_VERSION_ASYNCHDNS    (1<<7)  /* Asynchronous DNS resolves */
#define CURL_VERSION_SPNEGO       (1<<8)  /* SPNEGO auth is supported */
#define CURL_VERSION_LARGEFILE    (1<<9)  /* Supports files larger than 2GB */
#define CURL_VERSION_IDN          (1<<10) /* Internationized Domain Names are
                                             supported */
#define CURL_VERSION_SSPI         (1<<11) /* Built against Windows SSPI */
#define CURL_VERSION_CONV         (1<<12) /* Character conversions supported */
#define CURL_VERSION_CURLDEBUG    (1<<13) /* Debug memory tracking supported */
#define CURL_VERSION_TLSAUTH_SRP  (1<<14) /* TLS-SRP auth is supported */
#define CURL_VERSION_NTLM_WB      (1<<15) /* NTLM delegation to winbind helper
                                             is suported */
#define CURL_VERSION_HTTP2        (1<<16) /* HTTP2 support built-in */
#define CURL_VERSION_GSSAPI       (1<<17) /* Built against a GSS-API library */
#define CURL_VERSION_KERBEROS5    (1<<18) /* Kerberos V5 auth is supported */
#define CURL_VERSION_UNIX_SOCKETS (1<<19) /* Unix domain sockets support */
#define CURL_VERSION_PSL          (1<<20) /* Mozilla's Public Suffix List, used
                                             for cookie domain verification */

Changed to a FR as we don't have these constants in the source code.

Changing back to docs problem, as these constants were added in PHP 7.3, just need to be documented.

Constant added at https://github.com/php/php-src/commit/1badfd8171119ef70a5d088068b9de88ba668f73.