added border case and docs

[AlxEnashi]

Description

Arreglado un caso de borde cuando se pedian las forced photometry de un un objeto sin fp.
Agregado contenido de fp a docs.

Related Issue or Feature

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.

Project coverage is 85.02%. Comparing base (9fa326a) to head (67c8784).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
alerce/search.py	0.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #49      +/-   ##
==========================================
- Coverage   85.45%   85.02%   -0.44%     
==========================================
  Files           7        7              
  Lines         392      394       +2     
==========================================
  Hits          335      335              
- Misses         57       59       +2     

Files with missing lines	Coverage Δ
alerce/search.py	82.85% <0.00%> (-2.44%)	⬇️

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