Made suggested updates to functions docs

[ekansa]

brief description of changes

Updates to docs about creating custom functions

issues addressed

(https://github.com/archesproject/arches-docs/issues/441)

further comments

I haven't actually designed any functions, so I'll need some guidance to make sure this is correct.

To see the updates:

1. Function hooks: https://arches.readthedocs.io/en/funcs_7_5/developing/extending/extensions/functions/#function-hooks
2. Function descriptions: https://arches.readthedocs.io/en/funcs_7_5/developing/extending/extensions/functions/#defining-the-function-s-details

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This box must be checked

• [x] the PR branch was originally made from the base branch

This box should be checked

• [x] after these changes the docs build locally without error

This box should only be checked you intend to follow through on it (we can do it on our end too)

• [x] I will cherry-pick all commits in this PR into other branches that should have them after this PR is merged

[ekansa]

@bferguso Just a quick ping to see if you could review this PR based on the issue you raise. Thanks!

[mradamcox]

@bferguso @ekansa FWIW, there is an explanation of how to configure the resource descriptors function in the "configuring" section: https://arches.readthedocs.io/en/stable/configuring/define-resource-descriptors/. May be worth linking to from within this new content, or updating.

[ekansa]

@mradamcox Thanks !!! I added a cross reference as you suggested.