improve description for hyperlink usage

[Danfro]

The documentation for openxlsx package does provide information on the function makeHyperlinkString and for writeForumula.

In the description for makeHyperlinkString a reference to writeForumula is given:

create Excel hyperlink string

Wrapper to create internal hyperlink string to pass to writeFormula()

But the documentation for writeForumula does not contain an example or further description on how to use the makeHyperlinkString function in conjunction with writeForumula.

[github-actions]

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