Add docs for assigning properties via subquery

[therealsilence]

• Update data modification doc
• Update subqueries doc

[codecov]

Codecov Report

Merging #2097 (decc342) into master (4349729) will decrease coverage by 0.13%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2097      +/-   ##
==========================================
- Coverage   96.95%   96.81%   -0.14%     
==========================================
  Files         151      151              
  Lines       30107    30107              
==========================================
- Hits        29190    29149      -41     
- Misses        917      958      +41     

Flag	Coverage Δ
linux	96.81% <ø> (-0.14%)	⬇️
linux_replay	?

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Impacted Files	Coverage Δ
synapse/tests/utils.py	93.77% <0.00%> (-2.12%)	⬇️
synapse/lib/hiveauth.py	96.10% <0.00%> (-0.75%)	⬇️
synapse/lib/view.py	94.18% <0.00%> (-0.70%)	⬇️
synapse/cortex.py	95.91% <0.00%> (-0.68%)	⬇️
synapse/lib/trigger.py	93.91% <0.00%> (-0.44%)	⬇️
synapse/lib/layer.py	98.13% <0.00%> (-0.07%)	⬇️

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[therealsilence]

fixed/updated