RFR: 8290834: Improve potentially confusing documentation on collection of profiling information [v3]

[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.

Julian Waters has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:

 - Merge branch 'profile' of https://github.com/TheShermanTanker/jdk into profile
 - Correct comment with respect to review
 - Update compilationPolicy.hpp
 - Minor comment cleanup
 - Merge remote-tracking branch 'upstream/master' into profile
 - Better clarify documentation

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Changes:
  - all: https://git.openjdk.org/jdk/pull/9598/files
  - new: https://git.openjdk.org/jdk/pull/9598/files/7e485ac4..fba6dd27

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9598&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9598&range=01-02

  Stats: 318 lines in 45 files changed: 213 ins; 20 del; 85 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