[IIS Access Log]: Update documentation for custom log processing

[muthu-mps]

• Enhancement

Proposed commit message

Add note on the custom access log format processing.

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.

Author's Checklist

• [ ]

Related issues

• Closes [IIS] Enhance the document on support for default & custom log formats #8102

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• Start Time: 2023-12-06T11:34:23.653+0000

• Duration: 15 min 31 sec

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Name	Metrics % (covered/total)	Diff
Packages	100.0% (2/2)	💚
Files	100.0% (2/2)	💚
Classes	100.0% (2/2)	💚
Methods	86.111% (31/36)	👍 13.889
Lines	96.143% (349/363)	👍 18.061
Conditionals	100.0% (0/0)	💚

[milan-elastic]

Looks good!

[devamanv]

Also it's good to add a small description for the Proposed commit message or just use "see title" if the title is enough. We should start following the template which says:

This text will be pasted into the squash dialog when the change is committed and will be a long term historical record of the change to help future contributors understand the change, please help them by making it clear and comprehensive, they may be you.

[devamanv]

LGTM

[shmsr]

Left one comment. Otherwise, LGTM.

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Package iis - 1.17.3 containing this change is available at https://epr.elastic.co/search?package=iis