Mark `createFlash` as deprecated and improve documentation

1. Mark createFlash as @deprecated and encourage the use of the newer, Pajama compliant, createAlert.

2. Improve the JSDoc parameter documentation:

• stricter type for options.type (createFlash) and options.variant (createAlert): instead of String it is now a union of constants ('success'|'warning'...)
• add a missing parameter, options.addBodyClass, to createFlash
• change the way to indicate optional parameters: instead of using the ?, use [...] (see JSDoc).

Improved developer experience

vscode shows that the function is deprecated (crossed) and the possible values the option type:

In vscode, if the option "checkJs": true in jsconfig.json it will display an error for non-valid argument types: