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

[Ludvig Janiuk]

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

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Commit messages:
 - How to edit doc/testing, doc/building

Changes: https://git.openjdk.org/jdk/pull/10114/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10114&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293202
  Stats: 18 lines in 4 files changed: 18 ins; 0 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