docs: Help migrate with default features

Author: epage

CHANGELOG.md

@@ -125,9 +125,10 @@ Steps:
 2. *If using Builder API*: Explicitly set the `arg.action(ArgAction::...)` on each argument (`StoreValue` for options and `IncOccurrences` for flags)
 3. Run `cargo check --features clap/deprecated` and resolve all deprecation warnings
 4. Upgrade to v4
-5. Resolve compiler errors
-6. Resolve behavior changes (see "subtle changes" under BREAKING CHANGES)
-7. *At your leisure:* resolve new deprecation notices
+5. *If `default-features = false`*, run `cargo add clap -F help,usage,error-context`
+6. Resolve compiler errors
+7. Resolve behavior changes (see "subtle changes" under BREAKING CHANGES)
+8. *At your leisure:* resolve new deprecation notices
 
 Example test (derive):
 ```rust
@@ -155,7 +156,7 @@ fn verify_cli() {
 }
 ```
 
-Note: the idomatic / recommended way of specifying different types of args in the Builder API has changed:
+Note: the idiomatic / recommended way of specifying different types of args in the Builder API has changed:
 
 Before
 ```rust
@@ -184,7 +185,7 @@ Subtle changes (i.e. compiler won't catch):
 - `mut_arg` can no longer be used to customize help and version arguments, instead disable them (`Command::disable_help_flag`, `Command::disable_version_flag`) and provide your own (#4056)
 - Removed lifetimes from `Command`, `Arg`, `ArgGroup`, and `PossibleValue`, assuming `'static`.  `string` feature flag will enable support for `String`s (#1041, #2150, #4223)
 - `arg!(--flag <value>)` is now optional, instead of required.  Add `.required(true)` at the end to restore the original behavior (#4206)
-- Added default feature flags, `help`, `usage` and `error-context` (#4236)
+- Added default feature flags, `help`, `usage` and `error-context`, requiring adding them back in if `default-features = false` (#4236)
 - *(parser)* Always fill in `""` argument for external subcommands to make it easier to distinguish them from built-in commands (#3263)
 - *(parser)* Short flags now have higher precedence than hyphen values with `Arg::allow_hyphen_values`, to be consistent with `Command::allow_hyphen_values` (#4187)
 - *(parser)* `Arg::value_terminator` must be its own argument on the CLI rather than being in a delimited list (#4025)