Filter introduction suggestions

[hunter2009]

After reading useage of filtering multiple times, I found some filter usage rules.
It had three key elements and two operators:
Elements list

• package-name
• dir-name
• git(commitId or branch)

Two operators

• ... ellipsis
• ^

element(except dir-name) and operators can be mixed, like this：<operators>{package-name}<operators>. It had 20 combinations like this:

only package-name:(just 1)
1.package-name

package-name with one operator(4 situations):
2. ...package-name
3. package-name...
4. ^package-name
5. package-name^

package-name with two operator(6 situations):
6. ...package-name...
7. ^package-name^
8. ...^package-name,
9. ...package-name^
10. package-name^...
11. ^package-name...

package-name with three operator(8 situations)
...balabala...

package-name with four operator(1 situations)
...^package-name^...

It's very interesting and easy to get all combinations. So I think filter can be grouped by elements and operators, it's a good way to learn.

[hunter2009]

@zkochan Do you have any suggestions? I'm going to submit a merge request to slightly adjust the filter command doc structure by elements and operators

[zkochan]

Feel free to improve the docs.