internal/report: document CVEMeta vs CVE
internal/report: document CVEMeta vs CVE

Document when report.CVEMeta vs report.CVE and report.Description should
be used.

Change-Id: Icf2d75b4701d8c7aad84e0d4d3e826adce0a2402
Reviewed-on: https://go-review.googlesource.com/c/vulndb/+/357030
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Run-TryBot: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Reviewed-by: Roland Shoemaker <0d270388f2f92757a5de0f4bd891d3b392c44c4f@golang.org>

golang/vuln,golang/vuln