Undocumented/unintended that doc comment strings can contain format arguments

[shepmaster]

/// a. {this_is_not_ok}
#[derive(argh::FromArgs)]
struct Demo {
    /// b. {nor_is_this}
    #[argh(option)]
    value: u8,
}

[dependencies]
argh = "0.1.7"

error: there is no argument named `this_is_not_ok`
 --> src/main.rs:2:10
  |
2 | #[derive(argh::FromArgs)]
  |          ^^^^^^^^^^^^^^
  |
  = note: did you intend to capture a variable `this_is_not_ok` from the surrounding scope?
  = note: to avoid ambiguity, `format_args!` cannot capture variables when the format string is expanded from a macro
  = note: this error originates in the derive macro `argh::FromArgs` (in Nightly builds, run with -Z macro-backtrace for more info)

error: there is no argument named `nor_is_this`
 --> src/main.rs:2:10
  |
2 | #[derive(argh::FromArgs)]
  |          ^^^^^^^^^^^^^^
  |
  = note: did you intend to capture a variable `nor_is_this` from the surrounding scope?
  = note: to avoid ambiguity, `format_args!` cannot capture variables when the format string is expanded from a macro
  = note: this error originates in the derive macro `argh::FromArgs` (in Nightly builds, run with -Z macro-backtrace for more info)