settings: clarify what each value means

Most importantly, clarify what it means for HTTPS and SSH to be supported.

include/git2/common.h

@@ -109,9 +109,27 @@ GIT_EXTERN(void) git_libgit2_version(int *major, int *minor, int *rev);
  * was compiled
  */
 typedef enum {
+  /**
+   * If set, libgit2 was built thread-aware and can be safely used from multiple
+   * threads.
+   */
 	GIT_FEATURE_THREADS	= (1 << 0),
+  /**
+   * If set, libgit2 was built with and linked against a TLS implementation.
+   * Custom TLS streams may still be added by the user to support HTTPS
+   * regardless of this.
+   */
 	GIT_FEATURE_HTTPS	= (1 << 1),
+  /**
+   * If set, libgit2 was built with and linked against libssh2. A custom
+   * transport may still be added by the user to support libssh2 regardless of
+   * this.
+   */
 	GIT_FEATURE_SSH		= (1 << 2),
+  /**
+   * If set, libgit2 was built with support for sub-second resolution in file
+   * modification times.
+   */
 	GIT_FEATURE_NSEC	= (1 << 3),
 } git_feature_t;