[Telemetry Query Language] Iterate on processing doc

[TylerHelmuth]

Description:
This PR continues to mature the processing.md doc.

• Updates the metric's virtual field to be in alignment with recent discussions/PRs.
• Introduces the start of expected function syntax.

[TylerHelmuth]

/cc @bogdandrutu @tigrannajaryan @anuraaga @djaglowski as interested parties of the transform processor/telemetry query language

[codecov]

Codecov Report

Merging #5356 (e36455b) into main (c368d49) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #5356      +/-   ##
==========================================
+ Coverage   90.60%   90.64%   +0.03%     
==========================================
  Files         190      190              
  Lines       11429    11422       -7     
==========================================
- Hits        10355    10353       -2     
+ Misses        853      850       -3     
+ Partials      221      219       -2     

Impacted Files	Coverage Δ
pdata/internal/common.go	91.82% <0.00%> (-0.75%)	⬇️
internal/configunmarshaler/defaultunmarshaler.go
...e/internal/configunmarshaler/defaultunmarshaler.go	100.00% <0.00%> (ø)
config/configtelemetry/configtelemetry.go	100.00% <0.00%> (+9.67%)	⬆️
service/config_provider.go	90.32% <0.00%> (+14.64%)	⬆️

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[anuraaga]

Thanks, good clarification