dmr isoform docs: clarify --plot constraints, fix --max-pvalue typo, document mutual exclusivity of plot filters

[SuhasSrinivasan]

Hi @ArtRand

Thank you for the great work on the new dmr isoform subcommand in v0.6.3 is a very useful addition.
While testing it I ran into four issues with the current documentation at intro_dmr_isoform.md that could cause confusion for new users.
Noting them here for a documentation fix.

1. --plot requires --gene-name or --gene-id, but this is not explicitly stated

The current example uses an inline comment (# <-- requires a gene name (or gene id to plot)) that is easy to overlook. Users running dmr isoform in whole-transcriptome mode (without --gene-name/--gene-id) who add --plot will receive this error with no clear explanation:

Error! --plot requires --gene-id or --gene-name

2. --max-pval and --min-score are plot-only options, but this is not stated

The "Filtering the number of methylation marks" section presents these as general output filters. In practice they are tightly coupled to --plot — providing either without --plot results in an error. Users trying to filter the output table in whole-transcriptome mode will hit this unexpectedly.

3. --max-pval and --min-score are mutually exclusive, but this is not documented

Using both flags together produces:

error: the argument '--min-score <MIN_SCORE>' cannot be used with '--max-pval <MAX_PVAL>'

This is not mentioned anywhere in the documentation or the help text for either flag.

4. Typo: --max-pvalue in the "Filtering" section should be --max-pval

The actual CLI flag is --max-pval (no trailing e), but the documentation currently reads --max-pvalue.

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I would be happy to open a PR with fixes for all four points if that would be useful.
Thanks again for continuing to develop this tool!