Clarify default alignment for table columns in markdown

Change-Id: Idc31e355477b6c75a156f1100a4db6a4c07f0a41

Documentation/markdown.md

diff --git a/Documentation/markdown.md b/Documentation/markdown.md
index a5d5ec1..958fed9 100644
--- a/Documentation/markdown.md
+++ b/Documentation/markdown.md
@@ -172,7 +172,9 @@
 Placing `:` in the separator line indicates how the column should be
 aligned.  A colon on the left side is a **left-aligned** column; a
 colon on the right-most side is **right-aligned**; a colon on both
-sides is **center-aligned**.
+sides is **center-aligned**. If no alignment is specified, the column
+is aligned with the default for HTML `<td>` tags (left-aligned by
+default unless overridden by css).
 
 Empty table cells are indicated by whitespace between the column
 dividers (`| |`) while multiple column cells omit the whitespace.