[docs] Clarify createTheme warning for future compatibility

[satendra03]

Closes #46418

What

This PR improves the warning message in the theming documentation related to the usage of createTheme() with multiple arguments.

The original message says:

"Only the first argument (options) is processed by the createTheme() function. If you want to actually merge two themes' options and create a new one based on them, you may want to deep merge the two options and provide them as a first argument to the createTheme() function."

However, in practice, createTheme() currently supports multiple arguments due to internal deep merging, which can confuse developers who observe behavior that contradicts the warning.

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Why

The updated warning:

• Preserves the existing explanation
• Adds a clarification that the current support for multiple arguments exists for backward compatibility
• Clearly advises developers to manually deep merge theme objects to ensure future-proof code

This resolves confusion raised in #46418, where users expected the function to reject multiple arguments based on the wording, but observed that it worked in practice.

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How

Updated the :::warning block in:
docs/data/material/customization/theming/theming.md

New content explicitly explains:

• The reason multiple arguments still work
• That the behavior may be removed in future versions
• The recommended approach of manual deep merging

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Final

Developers will now see a clearer, forward-looking warning that helps them avoid depending on potentially deprecated behavior.

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[mui-bot]

Netlify deploy preview

• docs/data/material/customization/theming/theming.md

Bundle size report

Bundle	Parsed Size	Gzip Size
@mui/material	0B(0.00%)	0B(0.00%)
@mui/lab	0B(0.00%)	0B(0.00%)
@mui/system	0B(0.00%)	0B(0.00%)
@mui/utils	0B(0.00%)	0B(0.00%)

Details of bundle changes

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[satendra03]

Hi @mapache-salvaje @siriwatknp ,
The PR is still open after the requested changes were made and approved, just wanted to check if there's anything else needed!!