List the macros that guard breaking changes planned to become the
default in version 4.0.0, and explain that 4.0.0 will be the sum of
these opt-in flags, which can be tried on the 3.x release train.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>
The community-maintained clang-tidy check
modernize-nlohmann-json-explicit-conversions rewrites implicit
conversions into explicit get<T>() calls, which is exactly the
preparation the docs ask for ahead of implicit conversions being
switched off by default. Mention it on the JSON_USE_IMPLICIT_CONVERSIONS
page and in the migration guide, as promised in discussion #4610.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>