docs: Add clarifications for ruler remote write config options

[avanish-vaghela]

What this PR does / why we need it:

The update provides better understanding of the ruler remote write config behaviour. If a user specifies an 'X-Scope-OrgID' header under the 'headers' section of RemoteWriteConfig, the config parser silently drops this. This is not currently clear to the user and can lead to confusion. This update aims to clarify this.

It also adds clarification about what tenant ID value will be added to the 'X-Scope-OrgID' header in the remote write request if 'add_org_id_header' config parameter is set to 'true'.

Which issue(s) this PR fixes:
N/A

Special notes for your reviewer:
None

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

[CLAassistant]

All committers have signed the CLA.

[chaudum]

The configuration docs are generated from code, specifically from the ruler config in pkg/ruler/config.go. You need to change the wording there and run make docs to generate the documentation.

[avanish-vaghela]

I have the updated the comments in the code.