Reference to the C / C++ language bindings for #126 is too strong

[rearnsha]

The fix for issue #126 added a cross reference to the C & C++ language bindings, but the result is now misleading. The original text was intended to permit other languages to have their own rules for bit-field layout, but the text now could be read as implying that the C & C++ rules must apply to other languages as well. It think the best solution is to move the link to a footnote, with the text that points to the C/C++ bindings. eg

[footnote] The C & C++ layout rules for bit-fields are defined in [xref].

This would replace the text added in #126