Add documentation to Get process plugin

Self-assigning for review.

+++ b/core/modules/migrate/src/Plugin/migrate/process/Get.php
@@ -7,9 +7,74 @@
+ * get also supports a list of source properties. If this list contains an empty
+ * element then the current value will be used. This makes it impossible to
+ * reach a source property with an empty string as its name.

I think an example of this would be useful. The explanation makes sense, but an example would be very helpful. I did a quick search in core and didn't find anywhere multiple source properties are used.

Hmm - well, when using the static map process plugin, using multiple source properties is a somewhat common thing...

Added an example of multiple sources, as suggested by @ultimike.

+++ b/core/modules/migrate/src/Plugin/migrate/process/Get.php
@@ -36,9 +36,25 @@
+ * If the list of source properties contains an empty element then the current
+ * value will be used. This makes it impossible to reach a source property with
+ * an empty string as its name.

I think we should expand on this a bit. Maybe an example? If this is just something defensive that the plugin does internally, maybe we should simply omit this passage.

For a non-English speaker, the docs could be clearer. What is it doing? It is getting a value from the Row. Let's describe it then put in an example.

Changes done as per comment #9 & also added interdiff.

Kicking back to NW until #8 is addressed.

Tried to improve the readability of the docs in response to #8. Also corrected an indentation error.

+++ b/core/modules/migrate/src/Plugin/migrate/process/Get.php
@@ -7,9 +7,87 @@
+ * Available configuration keys:
+ *   - source: Source property.

I think the properties have to be flush with the word "Available".

Other than that, this looks pretty good. RTBC from me once this nit is fixed.

Those indentations catch me out so often! Fixed now.

Committed and pushed 4ea838b to 8.4.x and 6e953b8 to 8.3.x. Thanks!