twister: Fix description of relationship between filter and allow/exclude

        not (right associative)

        all comparison operators (non-associative)

    arch_allow, arch_exclude, platform_allow, platform_exclude

    are all syntactic sugar for these expressions. For instance

        arch_exclude = x86 arc

    Is the same as:

        filter = not ARCH in ["x86", "arc"]

    The ':' operator compiles the string argument as a regular expression,

    and then returns a true value only if the symbol's value in the environment

    matches. For example, if CONFIG_SOC="stm32f107xc" then

    Would match it.

    Note that arch_allow, arch_exclude, platform_allow, platform_exclude

    are not just syntactic sugar for filter expressions. For instance

        arch_exclude = x86 arc

    Can appear at first glance to have a similar effect to

        filter = not ARCH in ["x86", "arc"]

    but unlike "filter", these cause platforms to be filtered already during the testplan

    generation. While "filter" does not exclue platforms at the testplan generation, and instead

    relies on the result of running the build configuration stage. That is, to evaluate the filter

    expression, cmake is run for that target, and then the filter evaluated as a gate for the

    build and run steps.

    Therefore filtering by using {platform|arch}_{exclude|allow} is much faster.

The set of test cases that actually run depends on directives in the testsuite

files and options passed in on the command line. If there is any confusion,

running with -v or examining the test plan report (testplan.json)