RFR: JDK-8277028: Use service type documentation as fallback for @provides

[Hannes Wallnöfer]

This is a simple change to display the first sentence of the service type description in the "Provides" section of a module page if the `@provides` javadoc tag does not contain a description. This is the same we handle entries in the "Uses" section when no description is available from the `@uses` tag. The rationale is that it is still more useful to provide generic information about the service type than nothing if no provider-specific information is available.

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Commit messages:
 - JDK-8277028: Use service type documentation as fallback for @provides

Changes: https://git.openjdk.java.net/jdk/pull/6387/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6387&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277028
  Stats: 36 lines in 2 files changed: 29 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6387.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6387/head:pull/6387

PR: https://git.openjdk.java.net/jdk/pull/6387