RFR: 8320410: Reflow markdown in building.md

[Magnus Ihse Bursie]

Reflow the markdown in building.md so line lengths of 80 is respected, wherever possible. 

Note that is is correct that the building.html file is unchanged (and indeed a sign that only markdown line breaks have been modified).

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Commit messages:
 - 8320410: Reflow markdown in building.md

Changes: https://git.openjdk.org/jdk/pull/16737/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16737&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320410
  Stats: 305 lines in 1 file changed: 13 ins; 9 del; 283 mod
  Patch: https://git.openjdk.org/jdk/pull/16737.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16737/head:pull/16737

PR: https://git.openjdk.org/jdk/pull/16737