ARROW-13886 [R] Expand documentation for decimal()

First attempt at this. Will be followed by:
* opening Jira tickets to:
    * deprecate `decimal()`
    * implement `decimal128()` and `decimal256()`
* expand unit tests both for data types and `Arrays`

Tickets generated:
* implement `decimal128()` - https://issues.apache.org/jira/browse/ARROW-14843
* implement `decimal256()` - https://issues.apache.org/jira/browse/ARROW-14844
* improve messaging around `Decimal128Type` & `Decimal256Type` in the C++ code - https://issues.apache.org/jira/browse/ARROW-14842

Closes #11758 from dragosmg/ARROW-13886_decimal_docs

Authored-by: Dragos Moldovan-Grünfeld <dragos.mold@gmail.com>
Signed-off-by: Nic Crane <thisisnic@gmail.com>

r/R/type.R

@@ -181,14 +181,33 @@ NestedType <- R6Class("NestedType", inherit = DataType)
 #' `bit64::integer64` object) by setting `options(arrow.int64_downcast =
 #' FALSE)`.
 #'
+#' `decimal()` creates a `decimal128` type. Arrow decimals are fixed-point
+#' decimal numbers encoded as a scalar integer. The `precision` is the number of
+#' significant digits that the decimal type can represent; the `scale` is the
+#' number of digits after the decimal point. For example, the number 1234.567
+#' has a precision of 7 and a scale of 3. Note that `scale` can be negative.
+#'
+#' As an example, `decimal(7, 3)` can exactly represent the numbers 1234.567 and
+#' -1234.567 (encoded internally as the 128-bit integers 1234567 and -1234567,
+#' respectively), but neither 12345.67 nor 123.4567.
+#'
+#' `decimal(5, -3)` can exactly represent the number 12345000 (encoded
+#' internally as the 128-bit integer 12345), but neither 123450000 nor 1234500.
+#' The `scale` can be thought of as an argument that controls rounding. When
+#' negative, `scale` causes the number to be expressed using scientific notation
+#' and power of 10.
+#'
 #' @param unit For time/timestamp types, the time unit. `time32()` can take
 #' either "s" or "ms", while `time64()` can be "us" or "ns". `timestamp()` can
 #' take any of those four values.
 #' @param timezone For `timestamp()`, an optional time zone string.
 #' @param byte_width byte width for `FixedSizeBinary` type.
 #' @param list_size list size for `FixedSizeList` type.
-#' @param precision For `decimal()`, precision
-#' @param scale For `decimal()`, scale
+#' @param precision For `decimal()`, the number of significant digits
+#'    the arrow `decimal` type can represent. The maximum precision for
+#'    `decimal()` is 38 significant digits.
+#' @param scale For `decimal()`, the number of digits after the decimal
+#'    point. It can be negative.
 #' @param type For `list_of()`, a data type to make a list-of-type
 #' @param ... For `struct()`, a named list of types to define the struct columns
 #'