docs(storage-classes): add section on default StorageClass (#41843)

* docs(storage-classes): add section on default StorageClass

- empty commit at the beginning for the squash
  - PR template asks the first commit to be named appropriately for squashes

* Update storage-classes.md

Add Note at the end about having a single default storage class is preferred and that Kubernetes does not prevent you from having multiple storage classes.

* Updated storage-classed.md

Updated storage-classed.md based on feedback.

* update storage-class.md

Updated storage-class.md based on feedback

* fix feedback and grammar

- consistently use StorageClass, as that's how it's mostly used in this doc and the PVC doc
  - except when it is plural, in that case "storage classes" is used
  - fix docs reference to use the proper title of the doc too, which also has it as one word
- consolidate two sentences together that both describe what a default StorageClass is for
  - `storageClassName` is the specific PVC field; use backticks and camelCase when describing it
- remove extra new line

- minor grammar and spacing fixes
  - more than one space was occassionally used
  - a cloud provider will similarly only ever set _one_ default StorageClass, so use singular instead of plural

* docs(storage-classes): add section on default StorageClass

- the PVC docs reference this and there is an administrator guide on how to change this, but no section within the central Storage Classes document
  - this has made it a bit difficult to explain how that a default StorageClass exists as the StorageClass docs themselves barely mention it
  - with a new section, this makes it very clear and links out to other existing docs on the topic

- based off a previous PR that went through some review cycles and added this section but was never merged

* update what happens when multiple defaults exist

- c.f. kubernetes/kubernetes@962235c#diff-77fd84defc818b9e6010794bf87e0eece0960dfeff196e19b7d84da8de15ddffR67
  - chooses the newest by default

* slight modifications according to the style guide

- simplify dynamic provisionining reference
  - it's specific, but very accurate, so I'm hesitant to remove more pieces of it

- simplify tense; this is still a "hypothetical" (if/when) scenario, so still not present tense, but simpler past perfect tense instead of future perfect tense
  - futher modifications welcome!

* use a relative path to the doc instead of a URL

Co-authored-by: Rey Lejano <[email protected]>

---------

Co-authored-by: Ju Lim (Red Hat) <[email protected]>
Co-authored-by: Rey Lejano <[email protected]>

Author: 3 people

content/en/docs/concepts/storage/storage-classes.md

@@ -57,6 +57,17 @@ mountOptions:
 volumeBindingMode: Immediate
 ```
 
+### Default StorageClass
+
+When a PVC does not specify a `storageClassName`, the default StorageClass is
+used. The cluster can only have one default StorageClass. If more than one
+default StorageClass is accidentally set, the newest default is used when the
+PVC is dynamically provisioned.
+
+For instructions on setting the default StorageClass, see
+[Change the default StorageClass](/docs/tasks/administer-cluster/change-default-storage-class/).
+Note that certain cloud providers may already define a default StorageClass.
+
 ### Provisioner
 
 Each StorageClass has a provisioner that determines what volume plugin is used