[BUG] Improve signature docs

[TonyBagnall]

fixes #2840

Two people have recently told me they were using signatures. This will tidy up signature docs, not sure if we can find the missing image though.

was scope creeping into changing the code reverted to just do the docs

I've removed the example that relied on a non existent image. Ive checked, it never existed and there are two issues in the other place noting its absence. I've added a link to a medium post and James Morill's thesis

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