Improved property conversion docs

pkg/yqlib/doc/usage/headers/properties.md

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+# Properties
+
+Encode to a property file (decode not yet supported). Line comments on value nodes will be copied across.
+
+By default, empty maps and arrays are not encoded - see below for an example on how to encode a value for these.

pkg/yqlib/doc/usage/properties.md

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+# Properties
+
+Encode to a property file (decode not yet supported). Line comments on value nodes will be copied across.
+
+By default, empty maps and arrays are not encoded - see below for an example on how to encode a value for these.
+
+## Encode properties
+Note that empty arrays and maps are not encoded by default.
+
+Given a sample.yml file of:
+```yaml
+# block comments don't come through
+person: # neither do comments on maps
+    name: Mike # comments on values appear
+    pets: 
+    - cat # comments on array values appear
+    food: [pizza] # comments on arrays do not
+emptyArray: []
+emptyMap: []
+
+```
+then
+```bash
+yq -o=props -I=0 '.' sample.yml
+```
+will output
+```properties
+# comments on values appear
+person.name = Mike
+
+# comments on array values appear
+person.pets.0 = cat
+person.food.0 = pizza
+```
+
+## Encode properties: no comments
+Given a sample.yml file of:
+```yaml
+# block comments don't come through
+person: # neither do comments on maps
+    name: Mike # comments on values appear
+    pets: 
+    - cat # comments on array values appear
+    food: [pizza] # comments on arrays do not
+emptyArray: []
+emptyMap: []
+
+```
+then
+```bash
+yq -o=props -I=0 '... comments = ""' sample.yml
+```
+will output
+```properties
+person.name = Mike
+person.pets.0 = cat
+person.food.0 = pizza
+```
+
+## Encode properties: include empty maps and arrays
+Use a yq expression to set the empty maps and sequences to your desired value.
+
+Given a sample.yml file of:
+```yaml
+# block comments don't come through
+person: # neither do comments on maps
+    name: Mike # comments on values appear
+    pets: 
+    - cat # comments on array values appear
+    food: [pizza] # comments on arrays do not
+emptyArray: []
+emptyMap: []
+
+```
+then
+```bash
+yq -o=props -I=0 '(.. | select( (tag == "!!map" or tag =="!!seq") and length == 0)) = ""' sample.yml
+```
+will output
+```properties
+# comments on values appear
+person.name = Mike
+
+# comments on array values appear
+person.pets.0 = cat
+person.food.0 = pizza
+emptyArray = 
+emptyMap = 
+```
+