[Feature Request]: Documentation improvements around data_extract_spec and get_extract_datanames

[llrs-roche]

Feature description

The documentation of get_extract_datanames is a bit obscure. Particularly I find "Fetches dataname slot per data_extract_spec from a list of data_extract_spec" hard to follow: it works in a single object or it needs a list of data_extract_spect objects?
In addition, one link to data_extract_spec() might be helpful, and vice versa, a link from data_extract_spec to get_extract_datanames might help new users finding these functions for the first time.

Last, adding an example to get_extract_datanames might make it more approachable.
A simple example with just one dataset that could be added (it would be also nice to add with multiple datasets):

adtte_filters <- filter_spec(
    vars = c("PARAMCD", "CNSR"),
    sep = "-",
    choices = c("OS-1" = "OS-1", "OS-0" = "OS-0", "PFS-1" = "PFS-1"),
    selected = "OS-1",
    multiple = FALSE,
    label = "Choose endpoint and Censor"
)

des <- data_extract_spec(
    dataname = "ADTTE",
    filter = adtte_filters,
    select = select_spec(
        choices = c("AVAL", "BMRKR1", "AGE"),
        selected = c("AVAL", "BMRKR1"),
        multiple = TRUE,
        fixed = FALSE,
        label = "Column"
    )
)
get_extract_datanames(des)

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[llrs-roche]

Related to: insightsengineering/nestdevs-tasks#57 It might not be the best time investment. Or documentation should be updated to point to the replacement appropriately.