substandard error messages from the docs build

The docs build uses an internal tool to "fix up" the output from pandoc. If there are errors in the input, the source of the error is reported as "<stdin>".

$ make docs
Building target 'docs' in configuration 'macosx-x64'
Generating JDK_API javadoc for 60 modules
<stdin>:236: bad html
<stdin>:236: bad html
<stdin>:773: bad html
<stdin>:773: bad html
Finished building target 'docs' in configuration 'macosx-x64'

It would be better if the tool indicated the actual file in error.