docs(dotnet): document explicit_projects' filesystem-backstop limitation

explicit_projects has no filesystem backstop and relies entirely on
dotnet_takes_value's allowlist being exhaustive; a real dotnet flag
missing from it (e.g. --property/-p, --collect, --diag, --settings,
--blame, --output/-o, --source, --sln) whose value ends in a project
extension would still be misread as an explicit project path, the same
class of bug three earlier commits in this branch each closed one flag
at a time. Same root cause as arg_tokenizer.rs's Msbuild
single-segment-path limitation (real MSBuild needs a stat() call to
resolve the analogous ambiguity) -- documented as a known, deliberate
tradeoff rather than fixed, since teaching this function to stat()
every candidate would be a materially different, slower design.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>