Lua filter docs: extend description of table fields

doc/lua-filter-types-and-objects.md

@@ -259,24 +259,33 @@ Raw content of a specified format.
 
 A table.
 
-caption:
+`caption`
-:   table caption
+:   table caption ([list] of [inlines])
 
-aligns:
+`aligns`
-:   alignments
+:   column alignments ([list] of [Alignment]s)
 
-widths:
+`widths`
-:   column widths
+:   column widths (number)
 
 `headers`
-:   header row
+:   header row ([list] of [table cells](#table-cell))
 
-rows:
+`rows`
-:   table rows
+:   table rows ([list] of [list]s of [table cells](#table-cell))
 
 `tag`, `t`
 :   the literal `Table` (string)
 
+A [table cell]{#table-cell} is a list of blocks.
+
+[Alignment]{#Alignment} is a string value indicating the
+horizontal alignment of a table column. `AlignLeft`,
+`AlignRight`, and `AlignCenter` leads cell content tob be
+left-aligned, right-aligned, and centered, respectively. The
+default alignment is `AlignDefault` (often equivalent to
+centered).
+
 ## Inline
 
 ### Cite