runtime/debug: ParseBuildInfo is undocumented

[dmitshur]

debug.ParseBuildInfo has no documentation. (https://go.dev/s/style#doc-comments and https://go.dev/doc/comment say that every exported identifier should have a doc comment.)

[gopherbot]

Change https://go.dev/cl/621837 mentions this issue: runtime/debug: document ParseBuildInfo and (*BuildInfo).String