prospective-parachains: respond with multiple backable candidates (#3160)

Fixes https://github.com/paritytech/polkadot-sdk/issues/3129

polkadot/roadmap/implementers-guide/src/node/backing/prospective-parachains.md

@@ -92,9 +92,10 @@ prospective validation data. This is unlikely to change.
     been backed.
   - Sent by the Backing Subsystem after it successfully imports a
     statement giving a candidate the necessary quorum of backing votes.
-- `ProspectiveParachainsMessage::GetBackableCandidate`
-  - Get a backable candidate hash along with its relay parent for a given parachain,
-    under a given relay-parent (leaf) hash, which is a descendant of given candidate hashes.
+- `ProspectiveParachainsMessage::GetBackableCandidates`
+  - Get the requested number of backable candidate hashes along with their relay parent for a given
+    parachain,under a given relay-parent (leaf) hash, which are descendants of given candidate
+    hashes.
   - Sent by the Provisioner when requesting backable candidates, when
     selecting candidates for a given relay-parent.
 - `ProspectiveParachainsMessage::GetHypotheticalFrontier`

polkadot/roadmap/implementers-guide/src/node/utility/provisioner.md

@@ -187,16 +187,16 @@ this process is a vector of `CandidateHash`s, sorted in order of their core inde
 
 #### Required Path
 
-Required path is a parameter for `ProspectiveParachainsMessage::GetBackableCandidate`, which the provisioner sends in
+Required path is a parameter for `ProspectiveParachainsMessage::GetBackableCandidates`, which the provisioner sends in
 candidate selection.
 
-An empty required path indicates that the requested candidate should be a direct child of the most recently included
+An empty required path indicates that the requested candidate chain should start with the most recently included
 parablock for the given `para_id` as of the given relay parent.
 
 In contrast, a required path with one or more entries prompts [prospective
 parachains](../backing/prospective-parachains.md) to step forward through its fragment tree for the given `para_id` and
-relay parent until the desired parablock is reached. We then select a direct child of that parablock to pass to the
-provisioner.
+relay parent until the desired parablock is reached. We then select the chain starting with the direct child of that
+parablock to pass to the provisioner.
 
 The parablocks making up a required path do not need to have been previously seen as included in relay chain blocks.
 Thus the ability to provision backable candidates based on a required path effectively decouples backing from inclusion.