Added pipe and length docs, fix pipe precedence

pkg/yqlib/doc/Assign.md

@@ -8,7 +8,7 @@ This will do a similar thing to the plain form, however, the RHS expression is r
 ## Create yaml file
 Running
 ```bash
-yq eval --null-input '(.a.b = "cat") | (.x = "frog")'
+yq eval --null-input '.a.b = "cat" | .x = "frog"'
 ```
 will output
 ```yaml
@@ -112,7 +112,7 @@ a:
 ```
 then
 ```bash
-yq eval '.a.[] | select(. == "apple") |= "frog"' sample.yml
+yq eval '(.a.[] | select(. == "apple")) = "frog"' sample.yml
 ```
 will output
 ```yaml
@@ -130,7 +130,7 @@ Given a sample.yml file of:
 ```
 then
 ```bash
-yq eval '.[] | select(. == "*andy") |= "bogs"' sample.yml
+yq eval '(.[] | select(. == "*andy")) = "bogs"' sample.yml
 ```
 will output
 ```yaml

pkg/yqlib/doc/Length.md

@@ -0,0 +1,54 @@
+Returns the lengths of the nodes. Length is defined according to the type of the node.
+
+## String length
+returns length of string
+
+Given a sample.yml file of:
+```yaml
+a: cat
+```
+then
+```bash
+yq eval '.a | length' sample.yml
+```
+will output
+```yaml
+3
+```
+
+## Map length
+returns number of entries
+
+Given a sample.yml file of:
+```yaml
+a: cat
+c: dog
+```
+then
+```bash
+yq eval 'length' sample.yml
+```
+will output
+```yaml
+2
+```
+
+## Array length
+returns number of elements
+
+Given a sample.yml file of:
+```yaml
+- 2
+- 4
+- 6
+- 8
+```
+then
+```bash
+yq eval 'length' sample.yml
+```
+will output
+```yaml
+4
+```
+

pkg/yqlib/doc/Pipe.md

@@ -0,0 +1,35 @@
+Pipe the results of an expression into another. Like the bash operator.
+
+## Simple Pipe
+Given a sample.yml file of:
+```yaml
+a:
+  b: cat
+```
+then
+```bash
+yq eval '.a | .b' sample.yml
+```
+will output
+```yaml
+cat
+```
+
+## Multiple updates
+Given a sample.yml file of:
+```yaml
+a: cow
+b: sheep
+c: same
+```
+then
+```bash
+yq eval '.a = "cat" | .b = "dog"' sample.yml
+```
+will output
+```yaml
+a: cat
+b: dog
+c: same
+```
+

pkg/yqlib/doc/headers/Length.md

@@ -0,0 +1 @@
+Returns the lengths of the nodes. Length is defined according to the type of the node.

pkg/yqlib/doc/headers/Pipe.md

@@ -0,0 +1 @@
+Pipe the results of an expression into another. Like the bash operator.