DOC Document changes to asset admin file size config

[GuySartorelli]

Description

Documents changes to asset admin file size config - and documents setting that config in general.

Issues

• Fix clobbering of the upload size validation silverstripe-framework#10058

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[michalkleiner]

Suggested change

	If you want different file upload limits in the asset admin than you have in your upload fields, you can use the [`AssetAdmin.max_upload_size`](api:SilverStripe\AssetAdmin\Controller\AssetAdmin->max_upload_size) configuration property. This accepts the same values that `Upload_Validator.default_max_file_size` does.
	If you want different file upload limits in the asset admin than you have in your upload fields, you can use the [`AssetAdmin.max_upload_size`](api:SilverStripe\AssetAdmin\Controller\AssetAdmin->max_upload_size) configuration property. This accepts the values in the same format as `Upload_Validator.default_max_file_size`.

[GuySartorelli]

Done