Review and update the `message_chk()` documentation of the `sprintf`-like types

[joethorley]

The set of sprintf-like types accepted by message_chk() has grown (%es and the %% escape in #293, alongside %n, %s, %r, %y and %ies) but the documentation has not been reviewed as a whole.

Points to review:

• The %y entry reads 'y' if n == 1 otherwise 'ie', which is only usable when followed by %s.
  Its relationship to %ies is not explained (see Deprecate %y in favour of %ies in message_chk() #313).
• The single left-to-right pass is described, but there is no worked example showing why %% escapes a literal % and why %es is not consumed by %s.
• A lone % that does not start a recognized type is left as is; this is stated but not shown in the examples.
• The examples use director%y%s throughout and do not demonstrate %ies at all.
• Only a single n applies to the whole message (message_chk() supports only one count per message #311); the documentation does not say so.
• The types are documented only in ?message_chk; err(), wrn() and msg() inherit the n parameter but a reader landing on ?err has no visible list of the types.
  Consider whether the section should be shared or cross-referenced.
• Nothing in the README or the vignettes covers message construction; consider whether it belongs in vignettes/chk.Rmd.

Outcome: a reviewed and consistent @section sprintf-like types: block with examples that exercise every type, and cross-references from ?err.