RFR: 8259485: Document need for short paths when building on Windows

[liach]

Though this content seems simple, it would be extremely frustrating to newcomers, especially when the build stalls at "cannot find valid visual studio installation" for no clear reason in logs at all.

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Commit messages:
 - 8259485: Document need for short paths when building on Windows

Changes: https://git.openjdk.java.net/jdk/pull/2043/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2043&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259485
  Stats: 15 lines in 2 files changed: 15 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2043.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2043/head:pull/2043

PR: https://git.openjdk.java.net/jdk/pull/2043