RFR: 8267633: Clarify documentation of (Doc)TreeScanner

[Pavel Rappo]

When a method is said to be called "on an object", this means that the object is a receiver. When a method is said to be called "with an object", this means that the object is a parameter.

To scan an instance of (Doc)Tree, the "scan" method is called on the instance of (Doc)TreeScanner with that instance of (Doc)Tree.

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Commit messages:
 - Change @link to @code similarly to DocTreePath
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/4174/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4174&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267633
  Stats: 10 lines in 3 files changed: 0 ins; 0 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4174.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4174/head:pull/4174

PR: https://git.openjdk.java.net/jdk/pull/4174