Update blackfriday dependency per #2994 (#3857)

Signed-off-by: PJ Eby <pje@telecommunity.com>

vendor/github.com/russross/blackfriday/doc.go

@@ -0,0 +1,32 @@
+// Package blackfriday is a Markdown processor.
+//
+// It translates plain text with simple formatting rules into HTML or LaTeX.
+//
+// Sanitized Anchor Names
+//
+// Blackfriday includes an algorithm for creating sanitized anchor names
+// corresponding to a given input text. This algorithm is used to create
+// anchors for headings when EXTENSION_AUTO_HEADER_IDS is enabled. The
+// algorithm is specified below, so that other packages can create
+// compatible anchor names and links to those anchors.
+//
+// The algorithm iterates over the input text, interpreted as UTF-8,
+// one Unicode code point (rune) at a time. All runes that are letters (category L)
+// or numbers (category N) are considered valid characters. They are mapped to
+// lower case, and included in the output. All other runes are considered
+// invalid characters. Invalid characters that preceed the first valid character,
+// as well as invalid character that follow the last valid character
+// are dropped completely. All other sequences of invalid characters
+// between two valid characters are replaced with a single dash character '-'.
+//
+// SanitizedAnchorName exposes this functionality, and can be used to
+// create compatible links to the anchor names generated by blackfriday.
+// This algorithm is also implemented in a small standalone package at
+// github.com/shurcooL/sanitized_anchor_name. It can be useful for clients
+// that want a small package and don't need full functionality of blackfriday.
+package blackfriday
+
+// NOTE: Keep Sanitized Anchor Name algorithm in sync with package
+//       github.com/shurcooL/sanitized_anchor_name.
+//       Otherwise, users of sanitized_anchor_name will get anchor names
+//       that are incompatible with those generated by blackfriday.