treewide: fix typos

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

src/basic/macro.h

@@ -14,7 +14,7 @@
 
 /* Note: on GCC "no_sanitize_address" is a function attribute only, on llvm it may also be applied to global
  * variables. We define a specific macro which knows this. Note that on GCC we don't need this decorator so much, since
- * our primary usecase for this attribute is registration structures placed in named ELF sections which shall not be
+ * our primary use case for this attribute is registration structures placed in named ELF sections which shall not be
  * padded, but GCC doesn't pad those anyway if AddressSanitizer is enabled. */
 #if HAS_FEATURE_ADDRESS_SANITIZER && defined(__clang__)
 #define _variable_no_sanitize_address_ __attribute__((__no_sanitize_address__))

src/basic/pidref.h

@@ -3,7 +3,7 @@
 
 #include "macro.h"
 
-/* An embeddable structure carrying a reference to a process. Supposed to be used when tracking processes continously. */
+/* An embeddable structure carrying a reference to a process. Supposed to be used when tracking processes continuously. */
 typedef struct PidRef {
         pid_t pid; /* always valid */
         int fd;    /* only valid if pidfd are available in the kernel, and we manage to get an fd */

src/basic/special.h

@@ -57,7 +57,7 @@
  * Rules regarding adding further high level targets like the above:
  *
  * - Be conservative, only add more of these when we really need
- *   them. We need strong usecases for further additions.
+ *   them. We need strong use cases for further additions.
  *
  * - When there can be multiple implementations running side-by-side,
  *   it needs to be a .target unit which can pull in all