[jdk11u-dev] RFR: 8259485: Document need for short paths when building on Windows

[George Adams]

Backports a simple docs change

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Commit messages:
 - Backport 2243a1703b81aba2da0a943fc486e34f635f662c

Changes: https://git.openjdk.org/jdk11u-dev/pull/1543/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=1543&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8259485
  Stats: 15 lines in 2 files changed: 15 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/1543.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev pull/1543/head:pull/1543

PR: https://git.openjdk.org/jdk11u-dev/pull/1543