tracing: Update documentation on tracepoint glob matching

b0f1a59a "tracing/filters: Use a different op for glob match" added
glob matching to tracepoint filter strings. It uses the ftrace function
tracing glob matching facility that allows for the wild card character (*)
to be used at the start and/or end of the matching string.

But the documentation still states that the filtering only allows for
exact string matches.

Cc: Li Zefan <lizefan@huawei.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

Documentation/trace/events.txt

@@ -187,9 +187,18 @@ The operators available for numeric fields are:
 
 And for string fields they are:
 
-==, !=
+==, !=, ~
 
-Currently, only exact string matches are supported.
+The glob (~) only accepts a wild card character (*) at the start and or
+end of the string. For example:
+
+  prev_comm ~ "*sh"
+  prev_comm ~ "sh*"
+  prev_comm ~ "*sh*"
+
+But does not allow for it to be within the string:
+
+  prev_comm ~ "ba*sh"   <-- is invalid
 
 5.2 Setting filters
 -------------------