RFR: 8323546: Clarify docs for Compiler.perfmap filename parameter, and other misc related jcmd doc cleanups

[Chris Plummer]

The jcmd docs for Compiler.perfmap currently say:

    - *filename*: (Optional) The name of the map file (STRING, no default value)

However, there is a default, so not only should that be made more clear in the above, but also some descriptive text as to how the default is generated should be added.

VM.cds has a similar issue, but already has the descriptive text, so just the "no default value" part needs to be fixed.

Another change needed is to consistently use *filename* (italics) instead of `filename` (monospace). Note this is how html formatting is done. For the man page formatting, *filename* does no formatting and `filename` is displayed in color if supported. Personally I prefer `filename`, but it seems that there is already a strong precedence for using italics in the *arguments* list. For example:

    *arguments*:

    - *flag name*: The name of the flag that you want to set (STRING, no
        default value)

    - *string value*: (Optional) The value that you want to set (STRING, no
        default value)

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Commit messages:
 - minor improvements related to Compiler.perfmap filename argument

Changes: https://git.openjdk.org/jdk/pull/17359/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17359&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8323546
  Stats: 11 lines in 1 file changed: 5 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/17359.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17359/head:pull/17359

PR: https://git.openjdk.org/jdk/pull/17359