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Document the definition-time rejection of uncoercible collection types (#2837)

#2817 made ArrayCoercer build its element coercer eagerly in the
constructor rather than memoizing it on first use, so that coercers --
shared across requests -- create no state at request time. A side effect
went unrecorded: an Array/Set whose element type Grape cannot coerce is
now rejected while the params block is evaluated, so an application that
boots today can fail to boot on 4.0 with

    ArgumentError: type Foo should support coercion via `[]`

Previously the element coercer was built on the first request that
supplied the parameter, so a declaration never exercised by a request --
a documentation-only type, which is what this breaks in practice -- never
raised at all; one that was exercised got the same ArgumentError swallowed
by the coercion validator into a generic 400 "is invalid".

The tightening is right: the non-collection form (type: Foo) has always
raised at definition time, and the collection form's leniency was an
accident of the lazy build rather than a supported behavior. But it is a
load-time break, so it needs an UPGRADING entry, which this adds -- with
the three fixes that actually work (a one-argument parse, coerce_with, or
dropping type: in favour of documentation:), and a warning that defining
self.[] only silences the load-time error while leaving the parameter
failing per request.

Also pin the behavior: nothing asserted it anywhere, so a future refactor
could revert it silently. The three "raises" examples fail against the
pre-#2817 lazy build and pass against the eager one.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

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