Remove -o option from help text for validate subcommand

[catamorphism]

Since the validate subcommand has no WebAssembly output and ignores the -o option, this removes the -o option from the help text.

Also add link to the validation algorithm and edit the help text slightly.

[alexcrichton]

One possible idea if you find it helpful, there could be a test at tests/cli/help-validate.wat with the contents:

;; RUN: validate -h

and then BLESS=1 would auto-generate that as tests/cli/help-validate.wat.stdout with the contents:

Validate a WebAssembly binary

Usage: wasm-tools validate [OPTIONS] [INPUT]

Arguments:
  [INPUT]  Input file to process

Options:
      --generate-dwarf <lines|full>  Optionally generate DWARF debugging information from
                                     WebAssembly text files
  -g                                 Shorthand for `--generate-dwarf full`
  -f, --features <FEATURES>          Comma-separated list of WebAssembly features to enable during
                                     validation
  -o, --output <OUTPUT>              Where to place output
  -v, --verbose...                   Use verbose output (-v info, -vv debug, -vvv trace)
      --color <COLOR>                Configuration over whether terminal colors are used in output
                                     [default: auto]
  -h, --help                         Print help (see more with '--help')

Examples:

    # Validate `foo.wasm` with the default Wasm feature proposals.
    $ wasm-tools validate foo.wasm

    # Validate `foo.wasm` with more verbose output
    $ wasm-tools validate -vv foo.wasm

    # Validate `fancy.wasm` with all Wasm feature proposals enabled.
    $ wasm-tools validate --features all fancy.wasm

    # Validate `mvp.wasm` with the original wasm feature set enabled.
    $ wasm-tools validate --features=wasm1 mvp.wasm
    $ wasm-tools validate --features=mvp mvp.wasm

and that way we could see the diff during PR review and the affect that clap updates have too

[catamorphism]

Good idea about the tests -- I started on this in #2323.