Wildcard use in reference patterns is unclear

These basic search patterns, which are stated in the documenation, do work:
```
$ conan search -r sbre-public '*'
Existing package recipes:

boost/1.64.0@sbre/nightly
boost/1.64.0@sbre/testing
libalmath/0.1@sbre/nightly
libalmath/0.1@sbre/testing
libqi/1.5.0@sbre/nightly
libqi/1.5.0@sbre/testing
pthreads-win32/2.9.1@sbre/nightly
pthreads-win32/2.9.1@sbre/testing
qiframework/0.1@sbre/nightly
qiframework/0.1@sbre/testing
qilang/0.1@sbre/nightly
qilang/0.1@sbre/testing
sbretoolchain/2.9.0@sbre/nightly
sbretoolchain/2.9.0@sbre/testing

$ conan search -r sbre-public 'boost/*'
Existing package recipes:

boost/1.64.0@sbre/nightly
boost/1.64.0@sbre/testing
```

Now let's try to search by version:

```
$ conan search -r sbre-public '*2.9.1*'
There are no packages matching the '*2.9.1*' pattern

$ conan search -r sbre-public '*2.9.1/*'
There are no packages matching the '*2.9.1/*' pattern

$ conan search -r sbre-public '*/2.9.1'
Existing package recipes:

pthreads-win32/2.9.1@sbre/nightly
pthreads-win32/2.9.1@sbre/testing

$ conan search -r sbre-public '*/2.9.1/*'
Existing package recipes:

pthreads-win32/2.9.1@sbre/nightly
pthreads-win32/2.9.1@sbre/testing

$ conan search -r sbre-public '*/2.9.1@*/*'
Existing package recipes:

pthreads-win32/2.9.1@sbre/nightly
pthreads-win32/2.9.1@sbre/testing
```

Oddly enough, these patterns do work:
- `*/2.9.1` which is a pattern that doesn't try to match user/channel
- `*/2.9.1/*` which looks wrong (missing `@username`)

`*/2.9.1@*/*` looks well-formed, and works as expected.

Now let's try to search by user name:

Those patterns work:
`*/*@sbre*`
`*/*@sbre/*`

But those don't:
`*@sbre*`
`*@sbre/*`

And `*sbre*` alone will only search on the package name, not on the user name.
```
$ conan search -r sbre-public '*sbre*'
Existing package recipes:

sbretoolchain/2.9.0@sbre/nightly
sbretoolchain/2.9.0@sbre/testing
```

Now here's my main issue: matching by channel
```
$ conan search -r sbre-public '*/nightly'
There are no packages matching the '*/nightly' pattern

$ conan search -r sbre-public '*sbre/nightly'
There are no packages matching the '*sbre/nightly' pattern

$ conan search -r sbre-public '*@sbre/nightly'
There are no packages matching the '*@sbre/nightly' pattern

$ conan search -r sbre-public '*0.1@sbre/nightly'
There are no packages matching the '*0.1@sbre/nightly' pattern

$ conan search -r sbre-public '*@sbre/nightly'
There are no packages matching the '*@sbre/nightly' pattern

$ conan search -r sbre-public '*/0.1@sbre/nightly'
Existing package recipes:

libalmath/0.1@sbre/nightly
qiframework/0.1@sbre/nightly
qilang/0.1@sbre/nightly

$ conan search -r sbre-public '*/*@sbre/nightly'
Existing package recipes:

boost/1.64.0@sbre/nightly
libalmath/0.1@sbre/nightly
libqi/1.5.0@sbre/nightly
pthreads-win32/2.9.1@sbre/nightly
qiframework/0.1@sbre/nightly
qilang/0.1@sbre/nightly
sbretoolchain/2.9.0@sbre/nightly

$ conan search -r sbre-public '*/*/nightly'
There are no packages matching the '*/*/nightly' pattern

$ conan search -r sbre-public '*/*@*/nightly'
Existing package recipes:

boost/1.64.0@sbre/nightly
libalmath/0.1@sbre/nightly
libqi/1.5.0@sbre/nightly
pthreads-win32/2.9.1@sbre/nightly
qiframework/0.1@sbre/nightly
qilang/0.1@sbre/nightly
sbretoolchain/2.9.0@sbre/nightly
```

This is all confusing, so matching syntax should be better documented (unless I overlooked something), but the syntax could be improved (even only for Conan 2.0). I don't understand for example why sometimes`*` can be greedy (as in `boost/*`), and other times not (as in `*/nightly`), and why it sometimes only matches the package name, and other times not.