merge: move doc to ll-merge.h

Move the related documentation from Documentation/technical/api-merge.txt
to ll-merge.h as it's easier for the developers to find the usage
information beside the code instead of looking for it in another doc file.

Only the ll-merge related doc is removed from
documentation/technical/api-merge.txt because this information will be
redundant and it'll be hard to keep it up to date and synchronized with
the documentation in ll-merge.h.

Signed-off-by: Heba Waly <heba.waly@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ll-merge.h

@@ -7,16 +7,87 @@
 
 #include "xdiff/xdiff.h"
 
+/**
+ *
+ * Calling sequence:
+ * ----------------
+ *
+ * - Prepare a `struct ll_merge_options` to record options.
+ *   If you have no special requests, skip this and pass `NULL`
+ *   as the `opts` parameter to use the default options.
+ *
+ * - Allocate an mmbuffer_t variable for the result.
+ *
+ * - Allocate and fill variables with the file's original content
+ *   and two modified versions (using `read_mmfile`, for example).
+ *
+ * - Call `ll_merge()`.
+ *
+ * - Read the merged content from `result_buf.ptr` and `result_buf.size`.
+ *
+ * - Release buffers when finished.  A simple
+ *   `free(ancestor.ptr); free(ours.ptr); free(theirs.ptr);
+ *   free(result_buf.ptr);` will do.
+ *
+ * If the modifications do not merge cleanly, `ll_merge` will return a
+ * nonzero value and `result_buf` will generally include a description of
+ * the conflict bracketed by markers such as the traditional `<<<<<<<`
+ * and `>>>>>>>`.
+ *
+ * The `ancestor_label`, `our_label`, and `their_label` parameters are
+ * used to label the different sides of a conflict if the merge driver
+ * supports this.
+ */
+
+
 struct index_state;
 
+/**
+ * This describes the set of options the calling program wants to affect
+ * the operation of a low-level (single file) merge.
+ */
 struct ll_merge_options {
+
+	/**
+	 * Behave as though this were part of a merge between common ancestors in
+	 * a recursive merge (merges of binary files may need to be handled
+	 * differently in such cases, for example). If a helper program is
+	 * specified by the `[merge "<driver>"] recursive` configuration, it will
+	 * be used.
+	 */
 	unsigned virtual_ancestor : 1;
-	unsigned variant : 2;	/* favor ours, favor theirs, or union merge */
+
+	/**
+	 * Resolve local conflicts automatically in favor of one side or the other
+	 * (as in 'git merge-file' `--ours`/`--theirs`/`--union`).  Can be `0`,
+	 * `XDL_MERGE_FAVOR_OURS`, `XDL_MERGE_FAVOR_THEIRS`,
+	 * or `XDL_MERGE_FAVOR_UNION`.
+	 */
+	unsigned variant : 2;
+
+	/**
+	 * Resmudge and clean the "base", "theirs" and "ours" files before merging.
+	 * Use this when the merge is likely to have overlapped with a change in
+	 * smudge/clean or end-of-line normalization rules.
+	 */
 	unsigned renormalize : 1;
+
+	/**
+	 * Increase the length of conflict markers so that nested conflicts
+	 * can be differentiated.
+	 */
 	unsigned extra_marker_size;
+
+	/* Extra xpparam_t flags as defined in xdiff/xdiff.h. */
 	long xdl_opts;
 };
 
+/**
+ * Perform a three-way single-file merge in core.  This is a thin wrapper
+ * around `xdl_merge` that takes the path and any merge backend specified in
+ * `.gitattributes` or `.git/info/attributes` into account.
+ * Returns 0 for a clean merge.
+ */
 int ll_merge(mmbuffer_t *result_buf,
 	     const char *path,
 	     mmfile_t *ancestor, const char *ancestor_label,