Improve title docs

[hxtomprice]

What does this PR do? (please provide any background)

Updates the documentation for booking requests to link to the reference document for the Title field.
We have an enum of valid titles which are mentioned in a separate document but they aren't the easiest thing to find.
The error messaging to partners when they pass an invalid title is also not useful so this should help partners to self serve when calling the API.

Any tech debt?

Another ticket will be raised to improve the error messaging when an invalid title is passed in.

Screenshots / Screencast

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