nix-locate --help doesn't explain what the option to -t mean

[ralismark]

nix-locate --help includes this line for the option --type:

  -t, --type <TYPE>        Only print matches for files that have this type. If the option is given multiple times, a file will be printed if it has any of the given types [possible values: r, x, d, s]

however, it's not explain what each of these values mean

[bennofs]

The types mean: (r)egular file, e(x)cutable, (d)irectory, (s)ymlink

We should update the help string to explain that

[figsoda]

#220