Update MD013/line-length to permit long link/image reference definitions in all modes because they can not be easily split.

doc/md013.md

@@ -25,11 +25,11 @@ up into multiple lines. To set a different maximum length for headings, use
 `code_block_line_length`
 
 This rule has an exception when there is no whitespace beyond the configured
-line length. This allows you to still include items such as long URLs without
-being forced to break them in the middle. To disable this exception, set the
-`strict` parameter to `true` to report an issue when any line is too long.
-To warn for lines that are too long and could be fixed but allow lines without
-spaces, set the `stern` parameter to `true`.
+line length. This allows you to include items such as long URLs without being
+forced to break them in the middle. To disable this exception, set the `strict`
+parameter to `true` and an issue will be reported when any line is too long. To
+warn for lines that are too long and could be fixed but allow long lines
+without spaces, set the `stern` parameter to `true`.
 
 For example (assuming normal behavior):
 
@@ -50,5 +50,9 @@ Code blocks are included in this rule by default since it is often a
 requirement for document readability, and tentatively compatible with code
 rules. Still, some languages do not lend themselves to short lines.
 
+Lines with link/image reference definitions are always exempted from this rule
+(even in `strict` mode) because there is generally no way to split such lines
+without breaking the URL.
+
 Rationale: Extremely long lines can be difficult to work with in some editors.
 More information: <https://cirosantilli.com/markdown-style-guide#line-wrapping>.