XQFO: Context item → value

[ChristianGruen]

Resulting from #129 (related: #755): Many rules in the XQFO spec still refer to the context item, which is currently defined as follows:

When the context value is a single item, it can also be referred to as the context item; when it is a single node, it can also be referred to as the context node.

We need to make clear what’s going to happen if the context value is not a single item:

• In many cases, this can simply be done by replacing “context item” with “context value”.
• In some cases (e.g. for fn:string#0), we should specify that an error is raised (usually XPTY0004) if the input is not a single item.