RFR: 8290834: Improve potentially confusing documentation on collection of profiling information

[Julian Waters]

Documentation on the MethodData object incorrectly states that it is used when profiling in tiers 0 and 1, when it only does so for tier 0 (Interpreter), while tier 1 (Fully optimizing C1) does not collect any profile data at all. Additionally, the description for the different execution tiers is slightly misleading, as it seems to imply that MethodData is used in tier 3 as well, when profiling with C1 is done through ciMethodData instead. This cleanup attempts to slightly better clarify how profiling is tied together between the Interpreter and C1, explain what MDO is an abbreviation for (MethodData object), and corrects the documentation for MethodData as well.

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Commit messages:
 - Update compilationPolicy.hpp
 - Minor comment cleanup
 - Better clarify documentation

Changes: https://git.openjdk.org/jdk/pull/9598/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9598&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290834
  Stats: 9 lines in 2 files changed: 6 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/9598.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9598/head:pull/9598

PR: https://git.openjdk.org/jdk/pull/9598