Update Textual def guidance based on JB feedback + import CellGuide descriptions

[dosumis]

1. We currently have two pieces of guidance text on textual definitions

• Editing guidelines - defining terms
• Textual definitions SOP - this advocates more expansive cell type definitions and includes the concept of an extended description - as an attempt to grapple with how to work with CellGuide description text. Extended descriptions can include species specific information and markers & more general context than required for a core definition.

@JABelfiore has worked on merging these two: https://docs.google.com/spreadsheets/d/1OdTphQB85X7Z29yNDWyLGAeZJAiowBGVk2V43yn93gs/edit?gid=0#gid=0

2. We have begun to wholesale import extended descriptions from CellGuide (see below for example). We also plan to generated new extended descriptions for new cell types as we add. These are inevitably redundant with text in the core definition. The textual definition SOP proposes that editors deal with this by dividing text between core and extended. However, this is probably too much burden on curators. Instead the new guidelines should allow the redundancy to remain. The editors job is to make sure that the core definition and the description do not clash.

AIM: reconcile this into one new piece of text, with examples. Once we have an initial version, it will be essential for editors to test it against real examples & workflows.