RFR: 8058924: FileReader(String) documentation is insufficient

[Brian Burkhalter]

Add a statement to the `java.io` package documentation clarifying how a `String` representing a _pathname string_ is interpreted in the package.

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Commit messages:
 - 8058924: FileReader(String) documentation is insufficient

Changes: https://git.openjdk.java.net/jdk/pull/7767/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7767&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8058924
  Stats: 6 lines in 1 file changed: 4 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7767.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7767/head:pull/7767

PR: https://git.openjdk.java.net/jdk/pull/7767