Add documentation to MachineName process plugin

Self-assigning for review.

1. +++ b/core/modules/migrate/src/Plugin/migrate/process/MachineName.php
   @@ -11,13 +11,26 @@
   + * The machine_name process plugin takes the source value, runs it over the
   

   s/source/input. And let's say either "runs it through the transliteration service" or "transliterates it".

2. +++ b/core/modules/migrate/src/Plugin/migrate/process/MachineName.php
   @@ -11,13 +11,26 @@
   + * or a letter with an underscore and removes duplicate underscores. It's very
   + * often followed by one of the deduplication plugins.
   

   Not sure if the last sentence is helpful or confusing. Maybe we should axe it?

Refined text

I reviewed this carefully in person at Boston sprint weekend. RTBC assuming the tests pass (and why wouldn't they). Thanks, @Winthropian!

+++ b/core/modules/migrate/src/Plugin/migrate/process/MachineName.php
@@ -11,13 +11,28 @@
+ * the transliteration service. This makes the source value lowercase, ¶
+ * replaces anything that is not a number or a letter with an underscore, ¶
...
+ * Letters will have language decorations and accents removed. ¶

Trailing whitespace.

Leaving RTBC in the hope that that can be fixed on commit.

Straightforward code documentation improvements can always go into any patch release, alpha, beta, or RC, so please always file them against the production branch (currently 8.3.x). Thanks!

This patch has some trailing whitespace that needs to be cleaned up.

Or what @tstoeckler said, yes. Sorry, missed the comment. In general, committers editing things on commit is undesirable, and in the case of this patch, which is not urgent, the patch author can learn from it for future patches also. :)

Removed whitespaces.

Removed whitespaces.

Nothing objectionable here. Thanks!

+++ b/core/modules/migrate/src/Plugin/migrate/process/MachineName.php
@@ -11,13 +11,28 @@
+ * If the value of foo in the source is áéí! then the destination value of bar
+ * will be aei_.

Hm, I thought I posted this feedback already but apparently I did not! Can we put single quotes around the example values? I got confused by it the first time I read it. Thanks!

Added single quotes as requested by @xjm.

Looks made right. Thanks, @Jo Fitzgerald.

Thanks all! Committed to 8.4.x and cherry-picked to 8.3.x.