RFR: 8293202 Document how to edit doc/testing, doc/building [v2]

[Ludvig Janiuk]

> Added note to doc/testing and doc/building about the appropriate make target to regenerate them.

Ludvig Janiuk has refreshed the contents of this pull request, and previous commits have been removed. Incremental views are not available.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/10114/files
  - new: https://git.openjdk.org/jdk/pull/10114/files/e616d68d..17283cfe

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10114&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10114&range=00-01

  Stats: 18 lines in 4 files changed: 0 ins; 18 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/10114.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10114/head:pull/10114

PR: https://git.openjdk.org/jdk/pull/10114