Expand "Watermark errors" page

[rodrigomadalozzo]

As a support engineer, I have the following suggestions based on my experience handling cases related to disk space and watermark issues - https://www.elastic.co/docs/troubleshoot/elasticsearch/fix-watermark-errors:

1. Remove the unnecessary period before the comma.
2. Add a new Context section with a clearer explanation of watermark levels, as the current page assumes readers already understand the differences between low, high, and flood-stage watermarks.
3. Include a Common causes of watermark errors section outlining the most frequent reasons these issues occur, to help users prevent them.
4. Add a Preventing watermark errors section with practical recommendations such as setting up disk usage monitoring alerts, optimizing shard sizes, and mainly, implement more restrictive ILM policies.

cc @stefnestor

Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

• Yes
• [X ] No

[github-actions]

Vale Linting Results

Summary: 2 suggestions found

💡 Suggestions (2)

File	Line	Rule	Message
troubleshoot/elasticsearch/fix-watermark-errors.md	32	Elastic.EmDashes	Don't put a space before or after a dash.
troubleshoot/elasticsearch/fix-watermark-errors.md	62	Elastic.WordChoice	Consider using 'refer to (if it's a document), view (if it's a UI element)' instead of 'See', unless the term is in the UI.

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[github-actions]

🔍 Preview links for changed docs

• troubleshoot/elasticsearch/fix-watermark-errors.md

[yetanothertw]

Thank you for these improvements, they look really good! 🪴

I've left a few comments, mostly all around links syntax. Let me know if you need any help with that.

[yetanothertw]

I think we can be a bit more specific with the subheading. Context is too generic. Perhaps something like:

Suggested change

	## Context
	## Disk-based shard allocation watermarks

[stefnestor]

@rodrigomadalozzo "context" already exists implicitly in the first couple intro paragraphs. Perhaps we should consider merging/supplementing this content into that to avoid the duplication.

[stefnestor]

@rodrigomadalozzo if you feel up for it, we might be able to do two KBs in one go. Supplementing the one you're doing, you could consider (not giving dive-in detail just referencing) the dangers of bad disk watermarks by adding a bullet point like

* {{es}} recommends using default watermark settings. Experts can override [the watermark thresholds and headroom](elasticsearch://reference/elasticsearch/configuration-reference/cluster-level-shard-allocation-routing-settings.md) but risk not giving enough disk for background processes such as force merge, not being right-sized to data ingestion rates vs ILM settings, and possibly `disk is full` errors if 100% disk is reached.