more manual spelling fixes

doc/src/Errors_messages.txt

doc/src/Howto_drude2.txt

doc/src/Howto_spherical.txt

doc/src/Intro_nonfeatures.txt

doc/src/Packages_details.txt

doc/src/Speed_intel.txt

doc/src/compute_basal_atom.txt

doc/src/compute_rigid_local.txt

doc/src/compute_ti.txt

doc/src/fix_drude_transform.txt

doc/src/fix_phonon.txt

doc/src/fix_property_atom.txt

doc/src/fix_qmmm.txt

doc/src/fix_srd.txt

doc/src/message.txt

doc/src/pair_atm.txt

doc/src/pair_dipole.txt

doc/src/pair_lj_soft.txt