CXX-3102 Remove use of BSONCXX_ENUM in public headers

[eramongodb]

Resolves CXX-3102.

The X macro headers under bsoncxx/v_noabi/enum/ have long cause trouble with Doxygen documentation.

These troubles have worsened following #1182, especially in enumerations:

These are in addition to numerous functions and enumerators which are not properly documented, e.g. k_need_element_type_k_* and k_cannot_append_* error codes:

as well as ctor overloads for bsoncxx::v_noabi::types::bson_value::value and bsoncxx::v_noabi::types::bson_value::view.

This PR proposes replacing all use of BSONCXX_ENUM (specifically the X macro headers themselves) in favor of fully expanding and properly documenting each BSON type and binary subtype. The X macro headers may still be used in internal code which does not require API documentation for convenience.

The examples shown above are fixed to looks as follows:

[eramongodb]

Aside: ENUM_VALUES_PER_LINE = 1 is responsible for the strictly columnar list of enumerators. This change can be reverted or tweaked if it is deemed too space-wasteful, e.g. with the default value of 4:

With the maximum value of 20:

Or disabled entirely (note: enumerator values are no longer displayed at all; unsure where the SHOW_ENUM_VALUES setting has gone?):

[eramongodb]

Opted to use ENUM_VALUES_PER_LINE = 0 in favor of \showenumvalues to generate enumerator values in description tables only for enumerations that most benefit from it.

e.g. applied to BSON types and subtypes:

but not applied to error codes: