Minor updates to PayID Documentation

[keefertaylor]

High Level Overview of Change

Resolve a TODO and link to a formal specification for network headers.

Context of Change

Post launch cleanup.

Type of Change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Refactor (non-breaking change that only restructures code)
• [ ] Tests (You added tests for code that already exists, or your new feature included in this PR)
• [x] Documentation Updates
• [ ] Release

Before / After

n/a

Test Plan

CI

[codecov[bot]]

Codecov Report

Merging #253 into master will increase coverage by 1.65%. The diff coverage is 95.07%.

@@            Coverage Diff             @@
##           master     #253      +/-   ##
==========================================
+ Coverage   88.03%   89.69%   +1.65%     
==========================================
  Files          42       42              
  Lines         861      941      +80     
==========================================
+ Hits          758      844      +86     
+ Misses        103       97       -6     

Impacted Files	Coverage Δ
...ingKit/PayID/JavaScript/JavaScriptPayIDUtils.swift	100.00% <ø> (ø)
XpringKit/PayID/XRPPayIDClient.swift	85.00% <82.60%> (+3.18%)	:arrow_up:
XpringKit/PayID/PayIDClient.swift	96.55% <95.65%> (+15.30%)	:arrow_up:
XpringKit/XRP/DefaultXRPClient.swift	98.60% <100.00%> (+1.43%)	:arrow_up:
...XRP/Model/XRPPayment+Org_Xrpl_Rpc_V1_Payment.swift	100.00% <100.00%> (ø)
...l/XRPTransaction+Org_Xrpl_Rpc_V1_Transaction.swift	96.00% <100.00%> (+0.44%)	:arrow_up:
XpringKit/XRP/ReliableSubmissionXRPClient.swift	94.23% <100.00%> (+0.11%)	:arrow_up:
XpringKit/XRP/XRPClient.swift	100.00% <100.00%> (ø)
XpringKit/Xpring/XpringClient.swift	100.00% <100.00%> (+5.55%)	:arrow_up:

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