Remove references to specialist document finders

Removes the following text:
"Search API is currently used to publish some finders which do not fit
the standard specialist document finder pattern. These are:"

This text used to be:

"Rummager is currently used to publish two specific finders which do
not fit the standard specialist document finder pattern. These are:"

"two specific finders" was renamed to "some finders" here:
1351ba0#diff-bcd2e3cfb9dbd15522ba6293e991ea0072ccf34d78518fae4d0af939f3a67199L3-L4

...but the number of finders was never more than two. And, in fact, the
first finder was removed from the list here:
5af4254#diff-bcd2e3cfb9dbd15522ba6293e991ea0072ccf34d78518fae4d0af939f3a67199L6
Then the second finder removed here:
699534d#diff-bcd2e3cfb9dbd15522ba6293e991ea0072ccf34d78518fae4d0af939f3a67199L6

...meaning that there are now _no_ specialist finders published by Search API.

docs/publishing-finders.md

@@ -1,9 +1,7 @@
 # Publishing finders
 
-Search API is currently used to publish some finders which do not fit
-the standard specialist document finder pattern. These are:
-
-For new finder content items, use the rake task `publishing_api:publish_finder`. For example:
+Search API is used to publish finders and their corresponding email signup content items.
+For example:
 
 ```
 FINDER_CONFIG=news_and_communications.yml EMAIL_SIGNUP_CONFIG=news_and_communications_email_signup.yml publishing_api:publish_finder