Provide complete list of shorthand expressions of `dft_functional` in the document

[ahxbcn]

Details

The documentation for the INPUT keyword dft_functional in both the latest and stable versions of the online manual currently requires users to read source code in xc_functional.cpp to determine the shorthand names for different exchange-correlation functionals. This approach can make it difficult for users to identify which functionals can be used directly with their shorthand forms.

Additionally, while the linked C++ file xc_functional.cpp remains consistent between the latest documentation and the develop branch, it differs from the LTS branch version, creating potential confusion.

I propose the following improvements:

• Explicitly list all exchange-correlation functionals that support direct shorthand usage in the documentation.
• For popular exchange-correlation functionals supported through LIBXC (such as R2SCAN), include their exact syntax and usage examples directly in the documentation to enhance user convenience.

Have you read the online manual http://abacus.deepmodeling.com/en/latest/

• Yes, I have read the online manual.

Task list for Issue attackers (only for developers)

• Identify the specific section of the documentation with the issue.
• Investigate the issue and determine the root cause.
• Research best practices and potential solutions for the identified issue.
• Update the documentation to address the issue, following the suggested improvement.
• Ensure the updated documentation adheres to the project's documentation standards.
• Test the updated documentation to ensure it is clear and accurate.
• Review and incorporate any relevant feedback from users or developers.
• Publish the updated documentation and notify the issue reporter.