docs: document that `vpath` and `json` macros are feature-gated #6013

Job	Run time
cargo-deny check (bans licenses sources)	27s
cargo-hack	1m 13s
cargo-deny check (advisories)	23s
dependencies-are-sorted	4s
check (examples)	32s
Spell Check with Typos	9s
external-types	31s
check-docs	20s
check (.)	31s
wasm32-unknown-unknown	19s
test-msrv	2m 44s
test-docs	1m 10s
test-versions (beta, .)	52s
test-nightly	1m 2s
test-versions (stable, .)	1m 4s
test-versions (beta, examples)	41s
armv5te-unknown-linux-musleabi	35s
test-versions (stable, examples)	34s
	13m 11s

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