Docs: Document the new `applyInput` step
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From the same changeset, document the step name changes: 

> The following `ModifierStep` classes have all dropped their `Step` suffix, these
`Modifier` classes now all run at runtime, and are thus no longer steps; they're
invoked as part of the new `applyInput()` (TODO: document) step:
>
> - `ModifierStep` &rArr; `Modifier`
> - `PgBooleanFilterStep` &rArr; `PgBooleanFilter`
> - `PgClassFilterStep` &rArr; `PgClassFilter`
> - `PgConditionCapableParentStep` &rArr; `PgConditionCapableParent`
> - `PgConditionLikeStep` &rArr; `PgConditionLike`
> - `PgConditionStepMode` &rArr; `PgConditionMode`
> - `PgConditionStep` &rArr; `PgCondition`
> - `PgManyFilterStep` &rArr; `PgManyFilter`
> - `PgOrFilterStep` &rArr; `PgOrFilter`
> - `PgTempTableStep` &rArr; `PgTempTable`
> - `SetterCapableStep` &rArr; `SetterCapable`
> - `SetterStep` &rArr; `Setter`
>
> (Interestingly, other than the removal of `placeholder` and the fact they deal
with runtime values rather than steps now, they're very similar to what they
were before.)

