[yaml/en] Use preferred style; add missing uses

* YAML allows literal tabs in content, but not indentation.
* Two space indent always preferred.
  * Note: YAML dumpers always use 2 space by default.
* '- ...' doesn't need extra indentation.
  * Note: YAML dumpers don't use extra indentation.
* There was no mention of single quoted strings. They are preferred
  and should be used except when double quote semantics are actually
  required. (Best practice).
* Add flow form example for sets: `{a, b, c}`
* Show collapsed form of seq-in-seq: `- - - foo`.

yaml.html.markdown

@@ -2,8 +2,8 @@
 language: yaml
 filename: learnyaml.yaml
 contributors:
-  - ["Adam Brenecki", "https://github.com/adambrenecki"]
-  - ["Suhas SG", "https://github.com/jargnar"]
+- [Adam Brenecki, 'https://github.com/adambrenecki']
+- [Suhas SG, 'https://github.com/jargnar']
 ---
 
 YAML is a data serialisation language designed to be directly writable and
@@ -11,7 +11,7 @@ readable by humans.
 
 It's a strict superset of JSON, with the addition of syntactically
 significant newlines and indentation, like Python. Unlike Python, however,
-YAML doesn't allow literal tab characters at all.
+YAML doesn't allow literal tab characters for indentation.
 
 ```yaml
 # Comments in YAML look like this.
@@ -32,8 +32,10 @@ boolean: true
 null_value: null
 key with spaces: value
 # Notice that strings don't need to be quoted. However, they can be.
-however: "A string, enclosed in quotes."
-"Keys can be quoted too.": "Useful if you want to put a ':' in your key."
+however: 'A string, enclosed in quotes.'
+'Keys can be quoted too.': "Useful if you want to put a ':' in your key."
+single quotes: 'have ''one'' escape pattern'
+double quotes: "have many: \", \0, \t, \u263A, \x0d\x0a == \r\n, and more."
 
 # Multiple-line strings can be written either as a 'literal block' (using |),
 # or a 'folded block' (using '>').
@@ -59,7 +61,7 @@ folded_style: >
 # COLLECTION TYPES #
 ####################
 
-# Nesting is achieved by indentation.
+# Nesting uses indentation. 2 space indent is preferred (but not required).
 a_nested_map:
   key: value
   another_key: Another Value
@@ -83,22 +85,26 @@ a_nested_map:
   - Real Madrid
 : [ 2001-01-01, 2002-02-02 ]
 
-# Sequences (equivalent to lists or arrays) look like this:
+# Sequences (equivalent to lists or arrays) look like this
+# (note that the '-' counts as indentation):
 a_sequence:
-    - Item 1
-    - Item 2
-    - 0.5 # sequences can contain disparate types.
-    - Item 4
-    - key: value
+- Item 1
+- Item 2
+- 0.5 # sequences can contain disparate types.
+- Item 4
+- key: value
   another_key: another_value
-    -
+-
   - This is a sequence
   - inside another sequence
+- - - Nested sequence indicators
+    - can be collapsed
 
 # Since YAML is a superset of JSON, you can also write JSON-style maps and
 # sequences:
 json_map: {"key": "value"}
 json_seq: [3, 2, 1, "takeoff"]
+and quotes are optional: {key: [3, 2, 1, takeoff]}
 
 #######################
 # EXTRA YAML FEATURES #
@@ -157,6 +163,7 @@ set:
   ? item1
   ? item2
   ? item3
+or: {item1, item2, item3}
 
 # Like Python, sets are just maps with null values; the above is equivalent to:
 set2: