Docs: Document the new `applyInput` step

[jemgillam]

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 ⇒ Modifier
• PgBooleanFilterStep ⇒ PgBooleanFilter
• PgClassFilterStep ⇒ PgClassFilter
• PgConditionCapableParentStep ⇒ PgConditionCapableParent
• PgConditionLikeStep ⇒ PgConditionLike
• PgConditionStepMode ⇒ PgConditionMode
• PgConditionStep ⇒ PgCondition
• PgManyFilterStep ⇒ PgManyFilter
• PgOrFilterStep ⇒ PgOrFilter
• PgTempTableStep ⇒ PgTempTable
• SetterCapableStep ⇒ SetterCapable
• SetterStep ⇒ 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.)