Cleanup jcmd docs for Compiler.perfmap and VM.cds filename parameter

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)