docs: how to custom service uri when use database-discovery

[Anilople]

What's the purpose of this PR

Add more document about database-discovery in #4595

Which issue(s) this PR fixes:

None

Brief changelog

Tell users how to custom service uri like they do with eureka.

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[codecov]

Codecov Report

Merging #4743 (5f373cc) into master (0bf1028) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #4743      +/-   ##
============================================
- Coverage     47.17%   47.14%   -0.04%     
+ Complexity     1658     1655       -3     
============================================
  Files           349      349              
  Lines         10691    10691              
  Branches       1062     1062              
============================================
- Hits           5044     5040       -4     
- Misses         5339     5345       +6     
+ Partials        308      306       -2     

Impacted Files	Coverage Δ
...framework/apollo/openapi/entity/ConsumerAudit.java	42.42% <0.00%> (-6.07%)	⬇️
...mework/apollo/openapi/service/ConsumerService.java	56.92% <0.00%> (-1.54%)	⬇️

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[nobodyiam]

LGTM