RFR: [doc-only] JDK-8227697,,Improve text in Taglet API spec for expected results with standard doclet

[Jonathan Gibbons]

Please review a doc-only change to improve the documentation regarding 
user--defined taglets.  There is no change in behavior.

The CSR for the change has been approved.
JBS: https://bugs.openjdk.java.net/browse/JDK-8227697
CSR: https://bugs.openjdk.java.net/browse/JDK-8228561
Webrev: http://cr.openjdk.java.net/~jjg/8228561-taglet-stddoclet/webrev.00/

-- Jon