make it easier to work with metadata by providing more default behaviors

src/DataAPI.jl

@@ -359,7 +359,7 @@ the `key`.
 $STYLE_INFO
 
 The returned dictionary may be freshly allocated on each call to `metadata` and
-is considered to be owned by `x` so it should be only used for reading data.
+is considered to be owned by `x` so it must not be modified.
 """
 function metadata(x::T; style::Bool=false) where {T}
     if !metadatasupport(T).read
@@ -429,7 +429,11 @@ function colmetadata end
 """
     colmetadata(x, [col]; style::Bool=false)
 
-Return a dictionary mapping all column metadata keys to metadata values
+If `col` is not passed return a dictionary mapping columns represented as
+`Symbol` that have associated metadata to dictionaries mapping all
+metadata keys to metadata values associated with table `x` for a given column.
+
+If `col` is passed a dictionary mapping all column metadata keys to metadata values
 associated with column `col` of table `x`. Throw an error if `x` does not
 support reading metadata for column `col` or column `col` is not present in `x`.
 
@@ -439,13 +443,8 @@ the `key`.
 
 $STYLE_INFO
 
-If `col` is not passed return a dictionary mapping columns represented as
-`Symbol` that have associated metadata to dictionaries dictionary mapping all
-metadata keys to metadata values associated with table `x` for a given column.
-
 The returned dictionary may be freshly allocated on each call to `colmetadata`
-and is considered to be owned by `x` so it should be only used for reading
-data.
+and is considered to be owned by `x` so it must not be modified.
 """
 function colmetadata(x::T, col; style::Bool=false) where {T}
     if !colmetadatasupport(T).read