Doc: IR: make <texi-node> node-text dynamic

documentation-lib.scm: change the name of the accessor to the 'text'
member variable of <texi-node> to static-text, and make node-text an
independent method. Now the following is possible:

document-translation.scm: For functionally generated node text,
override the node-text method, rather than setting the static 'text'
member variable for each translator and context documentation node.
Doc: IR: Make <translator-doc> double as <texi-item>

Each translator has a texinfo documentation node of its own, but part
of that data is also embedded into tables in context documentation
nodes.

Reflect this with a smidgen of multiple inheritance: <translator-doc>
can both be a <texi-node> and a <texi-item>.
Doc: IR: Implement <property-doc> classes

Prepare to encapsulate information on music, context and backend
properties in instances of <property-doc> subclasses. This shall make
it easier to enrich their documentation nodes with cross-references.

So far, document only context properties with this
infrastructure. Music and backend properties shall follow later.