treewide: fix typos

- mostly: usecase -> use case
- continously -> continuously
- single typos in docs/FILE_DESCRIPTOR_STORE.md

src/shared/dissect-image.c

@@ -1825,7 +1825,7 @@ int partition_pick_mount_options(
          * anymore (since in some cases the kernel implementations will refuse mounting when corrupted,
          * read-only and "norecovery" is specified). But I think for the case of automatically determined
          * mount options for loopback devices this is the right choice, since otherwise using the same
-         * loopback file twice even in read-only mode, is going to fail badly sooner or later. The usecase of
+         * loopback file twice even in read-only mode, is going to fail badly sooner or later. The use case of
          * making reuse of the immutable images "just work" is more relevant to us than having read-only
          * access that actually modifies stuff work on such image files. Or to say this differently: if
          * people want their file systems to be fixed up they should just open them in writable mode, where

src/shared/image-policy.h

@@ -54,7 +54,7 @@ struct ImagePolicy {
         PartitionPolicy policies[];          /* sorted by designator, hence suitable for binary search */
 };
 
-/* Default policies for various usecases */
+/* Default policies for various use cases */
 extern const ImagePolicy image_policy_allow;
 extern const ImagePolicy image_policy_deny;
 extern const ImagePolicy image_policy_ignore;

src/shared/install-file.c

@@ -18,7 +18,7 @@ int fs_make_very_read_only(int fd) {
 
         assert(fd >= 0);
 
-        /* Tries to make the specified fd "comprehensively" read-only. Primary usecase for this is OS images,
+        /* Tries to make the specified fd "comprehensively" read-only. Primary use case for this is OS images,
          * i.e. either loopback files or larger directory hierarchies. Depending on the inode type and
          * backing file system this means something different:
          *