Update documentation to reflect changes to Ascertain Pedigrees

[Viqsi]

This is just splitting out the docs update task from #261 - making that work is one thing, but the documentation can't be touched until that's finalized.

For reference, here's the suggested text Veronica provided as a guideline:

Text for manual and/or top of page:

Ascertaining Pedigrees is a two-step procedure: First, you assemble a set of index cases, or an "index cohort." This could be, e.g., the set of all individuals with BP1 or BP2; the set of all individuals with a 'yes' response to a custom variable you have created; etc.. If you have not already done so, you can define your index cohort using "Build a Cohort." Second, you define ascertainment criteria for including Pedigrees in your selected "ascertained cohort." For example, you might simply request all relatives of everyone in your index cohort, or, you might require at least 2 relatives meeting certain criteria. For example, you might require at least 2 additional individuals in each pedigree with BP1 or BP2; or, you might require at least 4 additional individuals/pedigree with any affective diagnosis; etc. Note that for criteria involving multiple variables you will need to first use Define a Custom Variable to create a variable to use in this step. Additionally, you have the option of specifying a minimum pedigree size.

[Viqsi]

Finished and fully deployed with MathematicalMedicine/diverweb@0722053