Consider unifying how rustc cli flag values are presented

[jieyouxu]

Not necessarily for this PR, but I think we should settle on a consistent way of listing out sets of alternatives like this across our options. We currently have

• Exhaustively listed in command (--crate-type [bin|lib|rlib|...])
• Listed in help in parentheses (INFO may be one of (all-target-specs-json|calling-conventions|...))
• Listed in help as a textual list (The KIND can be one of dependency, crate, native, framework, or all (the default).)
  With this being a fourth potential layout

Originally posted by @clubby789 in #140077 (comment)

[clubby789]

For options where output is going to be lengthier (emit, print), perhaps we could explain only the syntax, and leave a link to the relevant reference entry? Alternatively, we could gate full explanations behind --verbose (not sure if this would require refactoring?)

[xizheyin]

There are indeed some inconsistency.

I researched some command line help messages and found the following rules:

1. Often, pointed brackets<option1|option2|...> indicate required parameters.
2. [option1|option2|...] is a common way of representing options which is not nessary.
3. * item - description -Branch list with star is the list of options for which a detailed description applies

So I think there needs to be many places in the current help message where changing [] and () to <> would be clearer. (This also avoids []lists conflicting with TYPE[=FILE])

Reference:

1. cargo help:

Rust's package manager

Usage: cargo [+toolchain] [OPTIONS] [COMMAND]
       cargo [+toolchain] [OPTIONS] -Zscript <MANIFEST_RS> [ARGS]...

Options:
  -V, --version                  Print version info and exit
      --list                     List installed commands
      --explain <CODE>           Provide a detailed explanation of a rustc error message
  -v, --verbose...               Use verbose output (-vv very verbose/build.rs output)
  -q, --quiet                    Do not print cargo log messages
      --color <WHEN>             Coloring: auto, always, never
  -C <DIRECTORY>                 Change to DIRECTORY before doing anything (nightly-only)
      --locked                   Assert that `Cargo.lock` will remain unchanged
      --offline                  Run without accessing the network
      --frozen                   Equivalent to specifying both --locked and --offline
      --config <KEY=VALUE|PATH>  Override a configuration value
  -Z <FLAG>                      Unstable (nightly-only) flags to Cargo, see 'cargo -Z help' for details
  -h, --help                     Print help

2. gcc help: https://man7.org/linux/man-pages/man1/gcc.1.html
3. curl help: https://curl.se/docs/manpage.html

[xizheyin]

Alternatively, we could gate full explanations behind --verbose (not sure if this would require refactoring?)

That is reasonable to me.