docs: clarify path semantics when matching, fixes #3598

(cherry picked from commit 9b73d31a40ffa463eb62b9703c2d10a93d3f585c)

src/borg/archiver.py

@@ -1942,6 +1942,15 @@ class Archiver:
 
     helptext = collections.OrderedDict()
     helptext['patterns'] = textwrap.dedent('''
+        The path/filenames used as input for the pattern matching start from the
+        currently active recursion root. You usually give the recursion root(s)
+        when invoking borg and these can be either relative or absolute paths.
+
+        So, when you give `relative/` as root, the paths going into the matcher
+        will look like `relative/.../file.ext`. When you give `/absolute/` as root,
+        they will look like `/absolute/.../file.ext`. This is meant when we talk
+        about "full path" below.
+
         File patterns support these styles: fnmatch, shell, regular expressions,
         path prefixes and path full-matches. By default, fnmatch is used for
         ``--exclude`` patterns and shell-style is used for the experimental ``--pattern``
@@ -1961,8 +1970,8 @@ class Archiver:
             the path separator ('\\' for Windows and '/' on other systems) is not
             treated specially. Wrap meta-characters in brackets for a literal
             match (i.e. `[?]` to match the literal character `?`). For a path
-            to match a pattern, it must completely match from start to end, or
-            must match from the start to just before a path separator. Except
+            to match a pattern, the full path must match, or it must match
+            from the start of the full path to just before a path separator. Except
             for the root path, paths will never end in the path separator when
             matching is attempted.  Thus, if a given pattern ends in a path
             separator, a '\*' is appended before matching is attempted.
@@ -1976,7 +1985,7 @@ class Archiver:
 
         Regular expressions, selector `re:`
             Regular expressions similar to those found in Perl are supported. Unlike
-            shell patterns regular expressions are not required to match the complete
+            shell patterns regular expressions are not required to match the full
             path and any substring match is sufficient. It is strongly recommended to
             anchor patterns to the start ('^'), to the end ('$') or both. Path
             separators ('\\' for Windows and '/' on other systems) in paths are
@@ -1986,13 +1995,13 @@ class Archiver:
 
         Path prefix, selector `pp:`
             This pattern style is useful to match whole sub-directories. The pattern
-            `pp:/data/bar` matches `/data/bar` and everything therein.
+            `pp:root/somedir` matches `root/somedir` and everything therein.
 
         Path full-match, selector `pf:`
-            This pattern style is useful to match whole paths.
+            This pattern style is (only) useful to match full paths.
             This is kind of a pseudo pattern as it can not have any variable or
-            unspecified parts - the full, precise path must be given.
-            `pf:/data/foo.txt` matches `/data/foo.txt` only.
+            unspecified parts - the full path must be given.
+            `pf:root/file.ext` matches `root/file.txt` only.
 
             Implementation note: this is implemented via very time-efficient O(1)
             hashtable lookups (this means you can have huge amounts of such patterns