Improve documentation for `epi_slide`'s `f` parameter when it's a function with named arguments

[brookslogan]

edf = bind_rows(
  tibble(geo_value = "ak", time_value = as.Date("2020-01-01") + 1:10, x1=1:10, y=1:10 + rnorm(10L))
) %>%
  as_epi_df()

edf %>%
  group_by(geo_value) %>%
  epi_slide(function(x, method="qr", ...) tibble(model=list(lm(y ~ x1, x, method=method))), n=7L)

produces

An `epi_df` object, with metadata:
* geo_type  = state
* time_type = day
* as_of     = 2022-04-05 12:05:18

# A tibble: 10 × 5
# Groups:   geo_value [1]
   geo_value time_value    x1      y slide_value_model
 * <chr>     <date>     <int>  <dbl> <list>           
 1 ak        2020-01-02     1 -0.128 <lm>             
 2 ak        2020-01-03     2  2.35  <lm>             
 3 ak        2020-01-04     3  2.65  <lm>             
 4 ak        2020-01-05     4  4.94  <lm>             
 5 ak        2020-01-06     5  4.00  <lm>             
 6 ak        2020-01-07     6  6.72  <lm>             
 7 ak        2020-01-08     7  6.33  <lm>             
 8 ak        2020-01-09     8  6.89  <lm>             
 9 ak        2020-01-10     9  8.66  <lm>             
10 ak        2020-01-11    10 10.0   <lm>             
Warning messages:
1: In lm(y ~ x1, x, method = method) :
  method = 'ak' is not supported. Using 'qr'
2: In lm(y ~ x1, x, method = method) :
  method = 'ak' is not supported. Using 'qr'
3: In lm(y ~ x1, x, method = method) :
  method = 'ak' is not supported. Using 'qr'
4: In lm(y ~ x1, x, method = method) :
  method = 'ak' is not supported. Using 'qr'
5: In lm(y ~ x1, x, method = method) :
  method = 'ak' is not supported. Using 'qr'
6: In lm(y ~ x1, x, method = method) :
  method = 'ak' is not supported. Using 'qr'
7: In lm(y ~ x1, x, method = method) :
  method = 'ak' is not supported. Using 'qr'
8: In lm(y ~ x1, x, method = method) :
  method = 'ak' is not supported. Using 'qr'
9: In lm(y ~ x1, x, method = method) :
  method = 'ak' is not supported. Using 'qr'
10: In lm(y ~ x1, x, method = method) :
  method = 'ak' is not supported. Using 'qr'

because the unnamed grouping-variables tibble (here, it'd be tibble(geo_value="ak") for our single group) is being passed in as the method argument to f, overriding the default qr (plus some "interesting" behavior of == and sprintf on tibbles going on inside lm when it checks it).

[brookslogan]

Fixes for the user in this case:

## Include a grouping-variables tibble row parameter:
edf %>%
  group_by(geo_value) %>%
  epi_slide(function(x,g, method="qr", ...) tibble(model=list(lm(y ~ x1, x, method=method))), n=7L)

## Pass in `method="qr"` to `epi_slide` --- no default args to `f` allowed:
edf %>%
  group_by(geo_value) %>%
  epi_slide(function(x, method="qr", ...) tibble(model=list(lm(y ~ x1, x, method=method))), n=7L, method="qr")

## Hide the named parameters of `f` behind `...` so they aren't matched by the
## positional grouping-variable tibble row argument:
edf %>%
  group_by(geo_value) %>%
  epi_slide(function(x, ..., method="qr") tibble(model=list(lm(y ~ x1, x, method=method))), n=7L)

The documentation should be revised to describe one of the above approaches.