Move doc comment about `GIT_BLOB_FILTER_OPTIONS_VERSION`

Removes doc comment on `git_blob_filter_options.version`, moves
information to `git_blob_filter_options` doc comment to remain
consistent with other options structures' documentation.

`git_blob_filter_options_init` still needed; should be added in another
commit/PR (it's out of the scope of this PR, #5759), update this
documentation again.

include/git2/blob.h

@@ -118,9 +118,11 @@ typedef enum {
 
 /**
  * The options used when applying filter options to a file.
+ *
+ * Initialize with `GIT_BLOB_FILTER_OPTIONS_VERSION`.
+ *
  */
 typedef struct {
-	/** The struct version; set to `GIT_BLOB_FILTER_OPTIONS_VERSION`. */
 	int version;
 
 	/** Flags to control the filtering process, see `git_blob_filter_flag_t` above */