docs: fix stale docstrings and export DEFAULT_MAX_* constants

match() docstring: qualified the round-trip claim with the RFC
section 1.4 caveat that values containing their operator's separator
unencoded do not round-trip (e.g. {.ext} with 'tar.gz').

resource() decorator docstring: removed the 'or the function has
parameters' clause which commit 674783f made stale; template/static
is now decided purely by URI variables.

Added DEFAULT_MAX_TEMPLATE_LENGTH, DEFAULT_MAX_EXPRESSIONS, and
DEFAULT_MAX_URI_LENGTH to __all__ to match the stated intent that
these are part of the public API.

Moved DEFAULT_MAX_URI_LENGTH import in test file from function body
to top-level per repo convention.