RFR: JDK-8185994: Fix a11y and HTML issues in the java.base/java.io and java.base/java.nio packages

[Jonathan Gibbons]

Please review this proposed change to address accessibility and HTML issues
in the java.base java.io and java.nio[.*] packages.

In general, the changes consist of
* update tables to add scope=row|col as appropriate.
     In some cases, slightly more surgery was needed on the table
* fix issues reported by tidy
     In most cases, this is removing superfluous <p> elements.

Webrev: http://cr.openjdk.java.net/~jjg/8185994/webrev.00/index.html
API: http://cr.openjdk.java.net/~jjg/8185994/api.00/index.html

-- Jon